SixtyPical Analysis
===================

This is a test suite, written in [Falderal][] format, for the SixtyPical
static analysis rules.

[Falderal]:     http://catseye.tc/node/Falderal

    -> Functionality "Analyze SixtyPical program" is implemented by
    -> shell command "bin/sixtypical --analyze-only --traceback %(test-body-file) && echo ok"

    -> Tests for functionality "Analyze SixtyPical program"

### Rudiments ###

Routines must declare their inputs, outputs, and memory locations they trash.

    | routine up
    |   inputs a
    |   outputs a
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     add a, 1
    | }
    = ok

Routines may not declare a memory location to be both an output and trashed.

    | routine main
    |   outputs a
    |   trashes a
    | {
    |     ld a, 0
    | }
    ? InconsistentConstraintsError: a

If a routine declares it outputs a location, that location should be initialized.

    | routine main
    |   outputs a, x, z, n
    | {
    |     ld x, 0
    | }
    ? UnmeaningfulOutputError: a

    | routine main
    |   inputs a
    |   outputs a
    | {
    | }
    = ok

If a routine declares it outputs a location, that location may or may not have
been initialized.  Trashing is mainly a signal to the caller.

    | routine main
    |   trashes x, z, n
    | {
    |     ld x, 0
    | }
    = ok

    | routine main
    |   trashes x, z, n
    | {
    | }
    = ok

If a routine modifies a location, it needs to either output it or trash it.

    | routine main
    | {
    |     ld x, 0
    | }
    ? ForbiddenWriteError: x

    | routine main
    |   outputs x, z, n
    | {
    |     ld x, 0
    | }
    = ok

    | routine main
    |   trashes x, z, n
    | {
    |     ld x, 0
    | }
    = ok

This is true regardless of whether it's an input or not.

    | routine main
    |   inputs x
    | {
    |     ld x, 0
    | }
    ? ForbiddenWriteError: x

    | routine main
    |   inputs x
    |   outputs x, z, n
    | {
    |     ld x, 0
    | }
    = ok

    | routine main
    |   inputs x
    |   trashes x, z, n
    | {
    |     ld x, 0
    | }
    = ok

If a routine trashes a location, this must be declared.

    | routine foo
    |   trashes x
    | {
    |     trash x
    | }
    = ok

    | routine foo
    | {
    |     trash x
    | }
    ? ForbiddenWriteError: x

    | routine foo
    |   outputs x
    | {
    |     trash x
    | }
    ? UnmeaningfulOutputError: x

If a routine causes a location to be trashed, this must be declared in the caller.

    | routine trash_x
    |   trashes x, z, n
    | {
    |   ld x, 0
    | }
    | 
    | routine foo
    |   trashes x, z, n
    | {
    |     call trash_x
    | }
    = ok

    | routine trash_x
    |   trashes x, z, n
    | {
    |   ld x, 0
    | }
    | 
    | routine foo
    |   trashes z, n
    | {
    |     call trash_x
    | }
    ? ForbiddenWriteError: x

    | routine trash_x
    |   trashes x, z, n
    | {
    |   ld x, 0
    | }
    | 
    | routine foo
    |   outputs x
    |   trashes z, n
    | {
    |     call trash_x
    | }
    ? UnmeaningfulOutputError: x (in foo, line 12)

If a routine reads or writes a user-define memory location, it needs to declare that too.

    | byte b1 @ 60000
    | byte b2 : 3
    | word w1 @ 60001
    | word w2 : 2000
    | 
    | routine main
    |   inputs b1, w1
    |   outputs b2, w2
    |   trashes a, z, n
    | {
    |   ld a, b1
    |   st a, b2
    |   copy w1, w2
    | }
    = ok

### ld ###

Can't `ld` from a memory location that isn't initialized.

    | routine main
    |   inputs a, x
    |   trashes a, z, n
    | {
    |     ld a, x
    | }
    = ok

    | routine main
    |   inputs a
    |   trashes a
    | {
    |     ld a, x
    | }
    ? UnmeaningfulReadError: x

Can't `ld` to a memory location that doesn't appear in (outputs ∪ trashes).

    | routine main
    |   trashes a, z, n
    | {
    |     ld a, 0
    | }
    = ok

    | routine main
    |   outputs a
    |   trashes z, n
    | {
    |     ld a, 0
    | }
    = ok

    | routine main
    |   outputs z, n
    |   trashes a
    | {
    |     ld a, 0
    | }
    = ok

    | routine main
    |   trashes z, n
    | {
    |     ld a, 0
    | }
    ? ForbiddenWriteError: a

    | routine main
    |   trashes a, n
    | {
    |     ld a, 0
    | }
    ? ForbiddenWriteError: z

Can't `ld` a `word` type.

    | word foo
    | 
    | routine main
    |   inputs foo
    |   trashes a, n, z
    | {
    |     ld a, foo
    | }
    ? TypeMismatchError: foo and a

### st ###

Can't `st` from a memory location that isn't initialized.

    | byte lives
    | routine main
    |   inputs x
    |   trashes lives
    | {
    |     st x, lives
    | }
    = ok

    | byte lives
    | routine main
    |   trashes x, lives
    | {
    |     st x, lives
    | }
    ? UnmeaningfulReadError: x

Can't `st` to a memory location that doesn't appear in (outputs ∪ trashes).

    | byte lives
    | routine main
    |   trashes lives
    | {
    |     st 0, lives
    | }
    = ok

    | byte lives
    | routine main
    |   outputs lives
    | {
    |     st 0, lives
    | }
    = ok

    | byte lives
    | routine main
    |   inputs lives
    | {
    |     st 0, lives
    | }
    ? ForbiddenWriteError: lives

Can't `st` a `word` type.

    | word foo
    | 
    | routine main
    |   outputs foo
    |   trashes a, n, z
    | {
    |     ld a, 0
    |     st a, foo
    | }
    ? TypeMismatchError

### tables ###

Storing to a table, you must use an index.

    | byte one
    | byte table[256] many
    | 
    | routine main
    |   outputs one
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     ld a, 0
    |     st a, one
    | }
    = ok

    | byte one
    | byte table[256] many
    | 
    | routine main
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     ld a, 0
    |     st a, many
    | }
    ? TypeMismatchError

    | byte one
    | byte table[256] many
    | 
    | routine main
    |   outputs one
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     ld a, 0
    |     st a, one + x
    | }
    ? TypeMismatchError

    | byte one
    | byte table[256] many
    | 
    | routine main
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     ld a, 0
    |     st a, many + x
    | }
    = ok

The index must be initialized.

    | byte one
    | byte table[256] many
    | 
    | routine main
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld a, 0
    |     st a, many + x
    | }
    ? UnmeaningfulReadError: x

Reading from a table, you must use an index.

    | byte one
    | 
    | routine main
    |   outputs one
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     st x, one
    |     ld a, one
    | }
    = ok

    | byte one
    | 
    | routine main
    |   outputs one
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     st x, one
    |     ld a, one + x
    | }
    ? TypeMismatchError

    | byte table[256] many
    | 
    | routine main
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     ld a, 0
    |     st a, many + x
    |     ld a, many
    | }
    ? TypeMismatchError

    | byte table[256] many
    | 
    | routine main
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     ld a, 0
    |     st a, many + x
    |     ld a, many + x
    | }
    = ok

    | byte table[256] many
    | 
    | routine main
    |   inputs many
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     ld a, many + x
    | }
    = ok

The index must be initialized.

    | byte table[256] many
    | 
    | routine main
    |   inputs many
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld a, many + x
    | }
    ? UnmeaningfulReadError: x

There are other operations you can do on tables. (1/3)

    | byte table[256] many
    | 
    | routine main
    |   inputs many
    |   outputs many
    |   trashes a, x, c, n, z, v
    | {
    |     ld x, 0
    |     ld a, 0
    |     st off, c
    |     add a, many + x
    |     sub a, many + x
    |     cmp a, many + x
    | }
    = ok

There are other operations you can do on tables. (2/3)

    | byte table[256] many
    | 
    | routine main
    |   inputs many
    |   outputs many
    |   trashes a, x, c, n, z
    | {
    |     ld x, 0
    |     ld a, 0
    |     and a, many + x
    |     or a, many + x
    |     xor a, many + x
    | }
    = ok

There are other operations you can do on tables. (3/3)

    | byte table[256] many
    | 
    | routine main
    |   inputs many
    |   outputs many
    |   trashes a, x, c, n, z
    | {
    |     ld x, 0
    |     ld a, 0
    |     st off, c
    |     shl many + x
    |     shr many + x
    |     inc many + x
    |     dec many + x
    | }
    = ok

Copying to and from a word table.

    | word one
    | word table[256] many
    | 
    | routine main
    |   inputs one, many
    |   outputs one, many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     copy one, many + x
    |     copy many + x, one
    | }
    = ok

    | word one
    | word table[256] many
    | 
    | routine main
    |   inputs one, many
    |   outputs one, many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     copy one, many
    | }
    ? TypeMismatchError

    | word one
    | word table[256] many
    | 
    | routine main
    |   inputs one, many
    |   outputs one, many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     copy one + x, many
    | }
    ? TypeMismatchError

You can also copy a literal word to a word table.
(Even if the table has fewer than 256 entries.)

    | word table[32] many
    | 
    | routine main
    |   inputs many
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     copy 9999, many + x
    | }
    = ok

#### tables: range checking ####

It is a static analysis error if it cannot be proven that a read or write
to a table falls within the defined size of that table.

(If a table has 256 entries, then there is never a problem, because a byte
cannot index any entry outside of 0..255.)

A SixtyPical implementation must be able to prove that the index is inside
the range of the table in various ways.  The simplest is to show that a
constant value falls inside or outside the range of the table.

    | byte table[32] many
    | 
    | routine main
    |   inputs many
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld x, 31
    |     ld a, many + x
    |     st a, many + x
    | }
    = ok

    | byte table[32] many
    | 
    | routine main
    |   inputs many
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld x, 32
    |     ld a, many + x
    | }
    ? RangeExceededError

    | byte table[32] many
    | 
    | routine main
    |   inputs many
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld x, 32
    |     ld a, 0
    |     st a, many + x
    | }
    ? RangeExceededError

This applies to `copy` as well.

    | word one: 77
    | word table[32] many
    | 
    | routine main
    |   inputs many, one
    |   outputs many, one
    |   trashes a, x, n, z
    | {
    |     ld x, 31
    |     copy one, many + x
    |     copy many + x, one
    | }
    = ok

    | word one: 77
    | word table[32] many
    | 
    | routine main
    |   inputs many, one
    |   outputs many, one
    |   trashes a, x, n, z
    | {
    |     ld x, 32
    |     copy many + x, one
    | }
    ? RangeExceededError

    | word one: 77
    | word table[32] many
    | 
    | routine main
    |   inputs many, one
    |   outputs many, one
    |   trashes a, x, n, z
    | {
    |     ld x, 32
    |     copy one, many + x
    | }
    ? RangeExceededError

`AND`'ing a register with a value ensures the range of the
register will not exceed the range of the value.  This can
be used to "clip" the range of an index so that it fits in
a table.

    | word one: 77
    | word table[32] many
    | 
    | routine main
    |   inputs a, many, one
    |   outputs many, one
    |   trashes a, x, n, z
    | {
    |     and a, 31
    |     ld x, a
    |     copy one, many + x
    |     copy many + x, one
    | }
    = ok

Test for "clipping", but not enough.

    | word one: 77
    | word table[32] many
    | 
    | routine main
    |   inputs a, many, one
    |   outputs many, one
    |   trashes a, x, n, z
    | {
    |     and a, 63
    |     ld x, a
    |     copy one, many + x
    |     copy many + x, one
    | }
    ? RangeExceededError

If you alter the value after "clipping" it, the range can
no longer be guaranteed.

    | word one: 77
    | word table[32] many
    | 
    | routine main
    |   inputs a, many, one
    |   outputs many, one
    |   trashes a, x, n, z
    | {
    |     and a, 31
    |     ld x, a
    |     inc x
    |     copy one, many + x
    |     copy many + x, one
    | }
    ? RangeExceededError

### add ###

Can't `add` from or to a memory location that isn't initialized.

    | routine main
    |   inputs a
    |   outputs a
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     add a, 0
    | }
    = ok

    | byte lives
    | routine main
    |   inputs a
    |   outputs a
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     add a, lives
    | }
    ? UnmeaningfulReadError: lives

    | byte lives
    | routine main
    |   inputs lives
    |   outputs a
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     add a, lives
    | }
    ? UnmeaningfulReadError: a

Can't `add` to a memory location that isn't writeable.

    | routine main
    |   inputs a
    |   trashes c
    | {
    |     st off, c
    |     add a, 0
    | }
    ? ForbiddenWriteError: a

You can `add` a word constant to a word memory location.

    | word score
    | routine main
    |   inputs a, score
    |   outputs score
    |   trashes a, c, z, v, n
    | {
    |     st off, c
    |     add score, 1999
    | }
    = ok

`add`ing a word constant to a word memory location trashes `a`.

    | word score
    | routine main
    |   inputs a, score
    |   outputs score, a
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     add score, 1999
    | }
    ? UnmeaningfulOutputError: a

To be sure, `add`ing a word constant to a word memory location trashes `a`.

    | word score
    | routine main
    |   inputs score
    |   outputs score
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     add score, 1999
    | }
    ? ForbiddenWriteError: a

You can `add` a word memory location to another word memory location.

    | word score
    | word delta
    | routine main
    |   inputs score, delta
    |   outputs score
    |   trashes a, c, z, v, n
    | {
    |     st off, c
    |     add score, delta
    | }
    = ok

`add`ing a word memory location to a word memory location trashes `a`.

    | word score
    | word delta
    | routine main
    |   inputs score, delta
    |   outputs score
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     add score, delta
    | }
    ? ForbiddenWriteError: a

You can `add` a word memory location, or a constant, to a pointer.

    | pointer ptr
    | word delta
    | routine main
    |   inputs ptr, delta
    |   outputs ptr
    |   trashes a, c, z, v, n
    | {
    |     st off, c
    |     add ptr, delta
    |     add ptr, word 1
    | }
    = ok

`add`ing a word memory location, or a constant, to a pointer, trashes `a`.

    | pointer ptr
    | word delta
    | routine main
    |   inputs ptr, delta
    |   outputs ptr
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     add ptr, delta
    |     add ptr, word 1
    | }
    ? ForbiddenWriteError: a

### sub ###

Can't `sub` from or to a memory location that isn't initialized.

    | routine main
    |   inputs a
    |   outputs a
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     sub a, 0
    | }
    = ok

    | byte lives
    | routine main
    |   inputs a
    |   outputs a
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     sub a, lives
    | }
    ? UnmeaningfulReadError: lives

    | byte lives
    | routine main
    |   inputs lives
    |   outputs a
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     sub a, lives
    | }
    ? UnmeaningfulReadError: a

Can't `sub` to a memory location that isn't writeable.

    | routine main
    |   inputs a
    |   trashes c
    | {
    |     st off, c
    |     sub a, 0
    | }
    ? ForbiddenWriteError: a

You can `sub` a word constant from a word memory location.

    | word score
    | routine main
    |   inputs a, score
    |   outputs score
    |   trashes a, c, z, v, n
    | {
    |     st on, c
    |     sub score, 1999
    | }
    = ok

`sub`ing a word constant from a word memory location trashes `a`.

    | word score
    | routine main
    |   inputs a, score
    |   outputs score, a
    |   trashes c, z, v, n
    | {
    |     st on, c
    |     sub score, 1999
    | }
    ? UnmeaningfulOutputError: a

You can `sub` a word memory location from another word memory location.

    | word score
    | word delta
    | routine main
    |   inputs score, delta
    |   outputs score
    |   trashes a, c, z, v, n
    | {
    |     st off, c
    |     sub score, delta
    | }
    = ok

`sub`ing a word memory location from a word memory location trashes `a`.

    | word score
    | word delta
    | routine main
    |   inputs score, delta
    |   outputs score
    |   trashes c, z, v, n
    | {
    |     st off, c
    |     sub score, delta
    | }
    ? ForbiddenWriteError: a

### inc ###

Location must be initialized and writeable.

    | routine main
    |   outputs x
    |   trashes z, n
    | {
    |     inc x
    | }
    ? UnmeaningfulReadError: x

    | routine main
    |   inputs x
    |   trashes z, n
    | {
    |     inc x
    | }
    ? ForbiddenWriteError: x

    | routine main
    |   inputs x
    |   outputs x
    |   trashes z, n
    | {
    |     inc x
    | }
    = ok

Can't `inc` a `word` type.

    | word foo
    | 
    | routine main
    |   inputs foo
    |   outputs foo
    |   trashes z, n
    | {
    |     inc foo
    | }
    ? TypeMismatchError: foo

### dec ###

Location must be initialized and writeable.

    | routine main
    |   outputs x
    |   trashes z, n
    | {
    |     dec x
    | }
    ? UnmeaningfulReadError: x

    | routine main
    |   inputs x
    |   trashes z, n
    | {
    |     dec x
    | }
    ? ForbiddenWriteError: x

    | routine main
    |   inputs x
    |   outputs x
    |   trashes z, n
    | {
    |     dec x
    | }
    = ok

Can't `dec` a `word` type.

    | word foo
    | 
    | routine main
    |   inputs foo
    |   outputs foo
    |   trashes z, n
    | {
    |     dec foo
    | }
    ? TypeMismatchError: foo

### cmp ###

Some rudimentary tests for `cmp`.

    | routine main
    |   inputs a
    |   trashes z, c, n
    | {
    |     cmp a, 4
    | }
    = ok

    | routine main
    |   inputs a
    |   trashes z, n
    | {
    |     cmp a, 4
    | }
    ? ForbiddenWriteError: c

    | routine main
    |   trashes z, c, n
    | {
    |     cmp a, 4
    | }
    ? UnmeaningfulReadError: a

### and ###

Some rudimentary tests for `and`.

    | routine main
    |   inputs a
    |   outputs a, z, n
    | {
    |     and a, 4
    | }
    = ok

    | routine main
    |   inputs a
    |   trashes z, n
    | {
    |     and a, 4
    | }
    ? ForbiddenWriteError: a

    | routine main
    |   trashes z, n
    | {
    |     and a, 4
    | }
    ? UnmeaningfulReadError: a

### or ###

Some rudimentary tests for `or`.

    | routine main
    |   inputs a
    |   outputs a, z, n
    | {
    |     or a, 4
    | }
    = ok

    | routine main
    |   inputs a
    |   trashes z, n
    | {
    |     or a, 4
    | }
    ? ForbiddenWriteError: a

    | routine main
    |   trashes z, n
    | {
    |     or a, 4
    | }
    ? UnmeaningfulReadError: a

### xor ###

Some rudimentary tests for `xor`.

    | routine main
    |   inputs a
    |   outputs a, z, n
    | {
    |     xor a, 4
    | }
    = ok

    | routine main
    |   inputs a
    |   trashes z, n
    | {
    |     xor a, 4
    | }
    ? ForbiddenWriteError: a

    | routine main
    |   trashes z, n
    | {
    |     xor a, 4
    | }
    ? UnmeaningfulReadError: a

### shl ###

Some rudimentary tests for `shl`.

    | byte foo
    | routine main
    |   inputs foo, a, c
    |   outputs foo, a, c, z, n
    | {
    |     shl a
    |     shl foo
    | }
    = ok

    | routine main
    |   inputs a, c
    |   outputs c, z, n
    | {
    |     shl a
    | }
    ? ForbiddenWriteError: a

    | routine main
    |   inputs a
    |   outputs a, c, z, n
    | {
    |     shl a
    | }
    ? UnmeaningfulReadError: c

### shr ###

Some rudimentary tests for `shr`.

    | byte foo
    | routine main
    |   inputs foo, a, c
    |   outputs foo, a, c, z, n
    | {
    |     shr a
    |     shr foo
    | }
    = ok

    | routine main
    |   inputs a, c
    |   outputs c, z, n
    | {
    |     shr a
    | }
    ? ForbiddenWriteError: a

    | routine main
    |   inputs a
    |   outputs a, c, z, n
    | {
    |     shr a
    | }
    ? UnmeaningfulReadError: c

### nop ###

Some rudimentary tests for `nop`.

    | routine main
    | {
    |     nop
    | }
    = ok

### call ###

When calling a routine, all of the locations it lists as inputs must be
initialized.

    | byte lives
    | 
    | routine foo
    |   inputs x
    |   trashes lives
    | {
    |     st x, lives
    | }
    | 
    | routine main
    | {
    |     call foo
    | }
    ? UnmeaningfulReadError: x

Note that if you call a routine that trashes a location, you also trash it.

    | byte lives
    | 
    | routine foo
    |   inputs x
    |   trashes lives
    | {
    |     st x, lives
    | }
    | 
    | routine main
    |   outputs x, z, n
    | {
    |     ld x, 0
    |     call foo
    | }
    ? ForbiddenWriteError: lives

    | byte lives
    | 
    | routine foo
    |   inputs x
    |   trashes lives
    | {
    |     st x, lives
    | }
    | 
    | routine main
    |   outputs x, z, n
    |   trashes lives
    | {
    |     ld x, 0
    |     call foo
    | }
    = ok

You can't output a value that the thing you called trashed.

    | byte lives
    | 
    | routine foo
    |   inputs x
    |   trashes lives
    | {
    |     st x, lives
    | }
    | 
    | routine main
    |   outputs x, z, n, lives
    | {
    |     ld x, 0
    |     call foo
    | }
    ? UnmeaningfulOutputError: lives

...unless you write to it yourself afterwards.

    | byte lives
    | 
    | routine foo
    |   inputs x
    |   trashes lives
    | {
    |     st x, lives
    | }
    | 
    | routine main
    |   outputs x, z, n, lives
    | {
    |     ld x, 0
    |     call foo
    |     st x, lives
    | }
    = ok

If a routine declares outputs, they are initialized in the caller after
calling it.

    | routine foo
    |   outputs x, z, n
    | {
    |     ld x, 0
    | }
    | 
    | routine main
    |   outputs a
    |   trashes x, z, n
    | {
    |     call foo
    |     ld a, x
    | }
    = ok

    | routine foo
    | {
    | }
    | 
    | routine main
    |   outputs a
    |   trashes x
    | {
    |     call foo
    |     ld a, x
    | }
    ? UnmeaningfulReadError: x

If a routine trashes locations, they are uninitialized in the caller after
calling it.

    | routine foo
    |   trashes x, z, n
    | {
    |     ld x, 0
    | }
    = ok

    | routine foo
    |   trashes x, z, n
    | {
    |     ld x, 0
    | }
    | 
    | routine main
    |   outputs a
    |   trashes x, z, n
    | {
    |     call foo
    |     ld a, x
    | }
    ? UnmeaningfulReadError: x

Calling an extern is just the same as calling a defined routine with the
same constraints.

    | routine chrout
    |   inputs a
    |   trashes a
    |   @ 65490
    | 
    | routine main
    |   trashes a, z, n
    | {
    |     ld a, 65
    |     call chrout
    | }
    = ok

    | routine chrout
    |   inputs a
    |   trashes a
    |   @ 65490
    | 
    | routine main
    |   trashes a, z, n
    | {
    |     call chrout
    | }
    ? UnmeaningfulReadError: a

    | routine chrout
    |   inputs a
    |   trashes a
    |   @ 65490
    | 
    | routine main
    |   trashes a, x, z, n
    | {
    |     ld a, 65
    |     call chrout
    |     ld x, a
    | }
    ? UnmeaningfulReadError: a

### trash ###

Trash does nothing except indicate that we do not care about the value anymore.

    | routine foo
    |   inputs a
    |   outputs x
    |   trashes a, z, n
    | {
    |     st a, x
    |     ld a, 0
    |     trash a
    | }
    = ok

    | routine foo
    |   inputs a
    |   outputs a, x
    |   trashes z, n
    | {
    |     st a, x
    |     ld a, 0
    |     trash a
    | }
    ? UnmeaningfulOutputError: a

    | routine foo
    |   inputs a
    |   outputs x
    |   trashes a, z, n
    | {
    |     st a, x
    |     trash a
    |     st a, x
    | }
    ? UnmeaningfulReadError: a

### if ###

Both blocks of an `if` are analyzed.

    | routine foo
    |   inputs a
    |   outputs x
    |   trashes a, z, n, c
    | {
    |     cmp a, 42
    |     if z {
    |         ld x, 7
    |     } else {
    |         ld x, 23
    |     }
    | }
    = ok

If a location is initialized in one block, is must be initialized in the other as well.

    | routine foo
    |   inputs a
    |   outputs x
    |   trashes a, z, n, c
    | {
    |     cmp a, 42
    |     if z {
    |         ld x, 7
    |     } else {
    |         ld a, 23
    |     }
    | }
    ? InconsistentInitializationError: x

    | routine foo
    |   inputs a
    |   outputs x
    |   trashes a, z, n, c
    | {
    |     cmp a, 42
    |     if z {
    |         ld a, 6
    |     } else {
    |         ld x, 7
    |     }
    | }
    ? InconsistentInitializationError: x

    | routine foo
    |   inputs a
    |   outputs x
    |   trashes a, z, n, c
    | {
    |     cmp a, 42
    |     if not z {
    |         ld a, 6
    |     } else {
    |         ld x, 7
    |     }
    | }
    ? InconsistentInitializationError: x

However, this only pertains to initialization.  If a value is already
initialized, either because it was set previous to the `if`, or is an
input to the routine, and it is initialized in one branch, it need not
be initialized in the other.

    | routine foo
    |   outputs x
    |   trashes a, z, n, c
    | {
    |     ld x, 0
    |     ld a, 0
    |     cmp a, 42
    |     if z {
    |         ld x, 7
    |     } else {
    |         ld a, 23
    |     }
    | }
    = ok

    | routine foo
    |   inputs x
    |   outputs x
    |   trashes a, z, n, c
    | {
    |     ld a, 0
    |     cmp a, 42
    |     if z {
    |         ld x, 7
    |     } else {
    |         ld a, 23
    |     }
    | }
    = ok

An `if` with a single block is analyzed as if it had an empty `else` block.

    | routine foo
    |   inputs a
    |   outputs x
    |   trashes a, z, n, c
    | {
    |     cmp a, 42
    |     if z {
    |         ld x, 7
    |     }
    | }
    ? InconsistentInitializationError: x

    | routine foo
    |   inputs a
    |   outputs x
    |   trashes a, z, n, c
    | {
    |     ld x, 0
    |     cmp a, 42
    |     if z {
    |         ld x, 7
    |     }
    | }
    = ok

    | routine foo
    |   inputs a
    |   outputs x
    |   trashes a, z, n, c
    | {
    |     ld x, 0
    |     cmp a, 42
    |     if not z {
    |         ld x, 7
    |     }
    | }
    = ok

The cardinal rule for trashes in an `if` is the "union rule": if one branch
trashes {`a`} and the other branch trashes {`b`} then the whole `if` statement
trashes {`a`, `b`}.

    | routine foo
    |   inputs a, x, z
    |   trashes a, x
    | {
    |     if z {
    |         trash a
    |     } else {
    |         trash x
    |     }
    | }
    = ok

    | routine foo
    |   inputs a, x, z
    |   trashes a
    | {
    |     if z {
    |         trash a
    |     } else {
    |         trash x
    |     }
    | }
    ? ForbiddenWriteError: x (in foo, line 10)

    | routine foo
    |   inputs a, x, z
    |   trashes x
    | {
    |     if z {
    |         trash a
    |     } else {
    |         trash x
    |     }
    | }
    ? ForbiddenWriteError: a (in foo, line 10)

### repeat ###

Repeat loop.

    | routine main
    |   outputs x, y, n, z, c
    | {
    |     ld x, 0
    |     ld y, 15
    |     repeat {
    |         inc x
    |         inc y
    |         cmp x, 10
    |     } until z
    | }
    = ok

You can initialize something inside the loop that was uninitialized outside.

    | routine main
    |   outputs x, y, n, z, c
    | {
    |     ld x, 0
    |     repeat {
    |         ld y, 15
    |         inc x
    |         cmp x, 10
    |     } until z
    | }
    = ok

But you can't UNinitialize something at the end of the loop that you need
initialized at the start.

    | routine foo
    |   trashes y
    | {
    | }
    | 
    | routine main
    |   outputs x, y, n, z, c
    | {
    |     ld x, 0
    |     ld y, 15
    |     repeat {
    |         inc x
    |         inc y
    |         call foo
    |         cmp x, 10
    |     } until z
    | }
    ? UnmeaningfulReadError: y

And if you trash the test expression (i.e. `z` in the below) inside the loop,
this is an error too.

    | word one : 0
    | word two : 0
    | 
    | routine main
    |   inputs one, two
    |   outputs two
    |   trashes a, z, n
    | {
    |     repeat {
    |         copy one, two
    |     } until z
    | }
    ? UnmeaningfulReadError: z

The body of `repeat forever` can be empty.

    | routine main
    | {
    |     repeat {
    |     } forever
    | }
    = ok

While `repeat` is most often used with `z`, it can also be used with `n`.

    | routine main
    |   outputs y, n, z
    | {
    |     ld y, 15
    |     repeat {
    |         dec y
    |     } until n
    | }
    = ok

### for ###

Basic "open-faced for" loop.  We'll start with the "upto" variant.

In a "for" loop, we know the exact range the loop variable takes on.

    | byte table[16] tab
    | 
    | define foo routine inputs tab trashes a, x, c, z, v, n {
    |     ld x, 0
    |     for x up to 15 {
    |         ld a, tab + x
    |     }
    | }
    = ok

    | byte table[15] tab
    | 
    | define foo routine inputs tab trashes a, x, c, z, v, n {
    |     ld x, 0
    |     for x up to 15 {
    |         ld a, tab + x
    |     }
    | }
    ? RangeExceededError

You need to initialize the loop variable before the loop.

    | byte table[16] tab
    | 
    | define foo routine inputs tab trashes a, x, c, z, v, n {
    |     for x up to 15 {
    |         ld a, 0
    |     }
    | }
    ? UnmeaningfulReadError

You cannot modify the loop variable in a "for" loop.

    | byte table[16] tab
    | 
    | define foo routine inputs tab trashes a, x, c, z, v, n {
    |     ld x, 0
    |     for x up to 15 {
    |         ld x, 0
    |     }
    | }
    ? ForbiddenWriteError

This includes nesting a loop on the same variable.

    | byte table[16] tab
    | 
    | define foo routine inputs tab trashes a, x, c, z, v, n {
    |     ld x, 0
    |     for x up to 8 {
    |         for x up to 15 {
    |             ld a, 0
    |         }
    |     }
    | }
    ? ForbiddenWriteError

But nesting with two different variables is okay.

    | byte table[16] tab
    | 
    | define foo routine inputs tab trashes a, x, y, c, z, v, n {
    |     ld x, 0
    |     for x up to 8 {
    |         ld a, x
    |         ld y, a
    |         for y up to 15 {
    |             ld a, 0
    |         }
    |     }
    | }
    = ok

Inside the inner loop, the outer variable is still not writeable.

    | byte table[16] tab
    | 
    | define foo routine inputs tab trashes a, x, y, c, z, v, n {
    |     ld x, 0
    |     for x up to 8 {
    |         ld a, x
    |         ld y, a
    |         for y up to 15 {
    |             ld x, 0
    |         }
    |     }
    | }
    ? ForbiddenWriteError

If the range isn't known to be smaller than the final value, you can't go up to it.

    | byte table[32] tab
    | 
    | define foo routine inputs tab trashes a, x, c, z, v, n {
    |     ld x, 16
    |     for x up to 15 {
    |         ld a, tab + x
    |     }
    | }
    ? RangeExceededError

In a "for" loop (downward-counting variant), we know the exact range the loop variable takes on.

    | byte table[16] tab
    | 
    | define foo routine inputs tab trashes a, x, c, z, v, n {
    |     ld x, 15
    |     for x down to 0 {
    |         ld a, tab + x
    |     }
    | }
    = ok

    | byte table[15] tab
    | 
    | define foo routine inputs tab trashes a, x, c, z, v, n {
    |     ld x, 15
    |     for x down to 0 {
    |         ld a, tab + x
    |     }
    | }
    ? RangeExceededError

You need to initialize the loop variable before a "for" loop  (downward variant).

    | byte table[16] tab
    | 
    | define foo routine inputs tab trashes a, x, c, z, v, n {
    |     for x down to 15 {
    |         ld a, 0
    |     }
    | }
    ? UnmeaningfulReadError

You cannot modify the loop variable in a "for" loop (downward variant).

    | byte table[16] tab
    | 
    | define foo routine inputs tab trashes a, x, c, z, v, n {
    |     ld x, 15
    |     for x down to 0 {
    |         ld x, 0
    |     }
    | }
    ? ForbiddenWriteError

If the range isn't known to be larger than the final value, you can't go down to it.

    | byte table[32] tab
    | 
    | define foo routine inputs tab trashes a, x, c, z, v, n {
    |     ld x, 0
    |     for x down to 0 {
    |         ld a, tab + x
    |     }
    | }
    ? RangeExceededError

You can initialize something inside the loop that was uninitialized outside.

    | routine main
    |   outputs x, y, n, z
    |   trashes c
    | {
    |     ld x, 0
    |     for x up to 15 {
    |         ld y, 15
    |     }
    | }
    = ok

But you can't UNinitialize something at the end of the loop that you need
initialized at the start of that loop.

    | routine foo
    |   trashes y
    | {
    | }
    | 
    | routine main
    |   outputs x, y, n, z
    |   trashes c
    | {
    |     ld x, 0
    |     ld y, 15
    |     for x up to 15 {
    |         inc y
    |         call foo
    |     }
    | }
    ? UnmeaningfulReadError: y

### save ###

Basic neutral test, where the `save` makes no difference.

    | routine main
    |   inputs a, x
    |   outputs a, x
    |   trashes z, n
    | {
    |     ld a, 1
    |     save x {
    |         ld a, 2
    |     }
    |     ld a, 3
    | }
    = ok

Saving any location (other than `a`) will trash `a`.

    | routine main
    |   inputs a, x
    |   outputs a, x
    |   trashes z, n
    | {
    |     ld a, 1
    |     save x {
    |         ld a, 2
    |     }
    | }
    ? UnmeaningfulOutputError

Saving `a` does not trash anything.

    | routine main
    |   inputs a, x
    |   outputs a, x
    |   trashes z, n
    | {
    |     ld x, 1
    |     save a {
    |         ld x, 2
    |     }
    |     ld x, 3
    | }
    = ok

A defined value that has been saved can be trashed inside the block.
It will continue to be defined outside the block.

    | routine main
    |   outputs x, y
    |   trashes a, z, n
    | {
    |     ld x, 0
    |     save x {
    |         ld y, 0
    |         trash x
    |     }
    | }
    = ok

A trashed value that has been saved can be used inside the block.
It will continue to be trashed outside the block.

    | routine main
    |   inputs a
    |   outputs a, x
    |   trashes z, n
    | {
    |     ld x, 0
    |     trash x
    |     save x {
    |         ld a, 0
    |         ld x, 1
    |     }
    | }
    ? UnmeaningfulOutputError: x

The known range of a value will be preserved outside the block as well.

    | word one: 77
    | word table[32] many
    | 
    | routine main
    |   inputs a, many, one
    |   outputs many, one
    |   trashes a, x, n, z
    | {
    |     and a, 31
    |     ld x, a
    |     save x {
    |         ld x, 255
    |     }
    |     copy one, many + x
    |     copy many + x, one
    | }
    = ok

    | word one: 77
    | word table[32] many
    | 
    | routine main
    |   inputs a, many, one
    |   outputs many, one
    |   trashes a, x, n, z
    | {
    |     and a, 63
    |     ld x, a
    |     save x {
    |         ld x, 1
    |     }
    |     copy one, many + x
    |     copy many + x, one
    | }
    ? RangeExceededError

The known properties of a value are preserved inside the block, too.

    | word one: 77
    | word table[32] many
    | 
    | routine main
    |   inputs a, many, one
    |   outputs many, one
    |   trashes a, x, n, z
    | {
    |     and a, 31
    |     ld x, a
    |     save x {
    |         copy one, many + x
    |         copy many + x, one
    |     }
    |     copy one, many + x
    |     copy many + x, one
    | }
    = ok

A value which is not output from the routine, is preserved by the
routine; and can appear in a `save` exactly because a `save` preserves it.

    | routine main
    |   outputs y
    |   trashes a, z, n
    | {
    |     save x {
    |         ld y, 0
    |         ld x, 1
    |     }
    | }
    = ok

Because saving anything except `a` trashes `a`, a common idiom is to save `a`
first in a nested series of `save`s.

    | routine main
    |   inputs a
    |   outputs a
    |   trashes z, n
    | {
    |     save a {
    |         save x {
    |             ld a, 0
    |             ld x, 1
    |         }
    |     }
    | }
    = ok

Not just registers, but also user-defined locations can be saved.

    | byte foo
    | 
    | routine main
    |   trashes a, z, n
    | {
    |     save foo {
    |         st 5, foo
    |     }
    | }
    = ok

But only if they are bytes.

    | word foo
    | 
    | routine main
    |   trashes a, z, n
    | {
    |     save foo {
    |         copy 555, foo
    |     }
    | }
    ? TypeMismatchError

    | byte table[16] tab
    | 
    | routine main
    |   trashes a, y, z, n
    | {
    |     save tab {
    |         ld y, 0
    |         st 5, tab + y
    |     }
    | }
    ? TypeMismatchError

A `goto` cannot appear within a `save` block, even if it is otherwise in tail position.

    | routine other
    |   trashes a, z, n
    | {
    |     ld a, 0
    | }
    | 
    | routine main
    |   trashes a, z, n
    | {
    |     ld a, 1
    |     save x {
    |         ld x, 2
    |         goto other
    |     }
    | }
    ? IllegalJumpError

### with interrupts ###

    | vector routine
    |   inputs x
    |   outputs x
    |   trashes z, n
    |     bar
    | 
    | routine foo
    |   inputs x
    |   outputs x
    |   trashes z, n
    | {
    |     inc x
    | }
    | 
    | routine main
    |   outputs bar
    |   trashes a, n, z
    | {
    |   with interrupts off {
    |     copy foo, bar
    |   }
    | }
    = ok

A `goto` cannot appear within a `with interrupts` block, even if it is
otherwise in tail position.

    | vector routine
    |   inputs x
    |   outputs x
    |   trashes z, n
    |     bar
    | 
    | routine foo
    |   inputs x
    |   outputs x
    |   trashes z, n
    | {
    |     inc x
    | }
    | 
    | routine other
    |   trashes bar, a, n, z
    | {
    |    ld a, 0
    | }
    | 
    | routine main
    |   trashes bar, a, n, z
    | {
    |   with interrupts off {
    |     copy foo, bar
    |     goto other
    |   }
    | }
    ? IllegalJumpError

### copy ###

Can't `copy` from a memory location that isn't initialized.

    | byte lives
    | routine main
    |   inputs x
    |   outputs lives
    |   trashes a, z, n
    | {
    |     copy x, lives
    | }
    = ok

    | byte lives
    | routine main
    |   outputs lives
    |   trashes x, a, z, n
    | {
    |     copy x, lives
    | }
    ? UnmeaningfulReadError: x

Can't `copy` to a memory location that doesn't appear in (outputs ∪ trashes).

    | byte lives
    | routine main
    |   trashes lives, a, z, n
    | {
    |     copy 0, lives
    | }
    = ok

    | byte lives
    | routine main
    |   outputs lives
    |   trashes a, z, n
    | {
    |     copy 0, lives
    | }
    = ok

    | byte lives
    | routine main
    |   inputs lives
    |   trashes a, z, n
    | {
    |     copy 0, lives
    | }
    ? ForbiddenWriteError: lives

a, z, and n are trashed, and must be declared as such

    | byte lives
    | routine main
    |   outputs lives
    | {
    |     copy 0, lives
    | }
    ? ForbiddenWriteError: n

a, z, and n are trashed, and must not be declared as outputs.

    | byte lives
    | routine main
    |   outputs lives, a, z, n
    | {
    |     copy 0, lives
    | }
    ? UnmeaningfulOutputError: n

Unless of course you subsequently initialize them.

    | byte lives
    | routine main
    |   outputs lives, a, z, n
    | {
    |     copy 0, lives
    |     ld a, 0
    | }
    = ok

Can `copy` from a `byte` to a `byte`.

    | byte source : 0
    | byte dest
    | 
    | routine main
    |   inputs source
    |   outputs dest
    |   trashes a, z, n
    | {
    |     copy source, dest
    | }
    = ok

The understanding is that, because `copy` trashes `a`, `a` cannot be used
as the destination of a `copy`.

    | byte source : 0
    | byte dest
    | 
    | routine main
    |   inputs source
    |   outputs dest
    |   trashes a, z, n
    | {
    |     copy source, a
    | }
    ? ForbiddenWriteError

Can `copy` from a `word` to a `word`.

    | word source : 0
    | word dest
    | 
    | routine main
    |   inputs source
    |   outputs dest
    |   trashes a, z, n
    | {
    |     copy source, dest
    | }
    = ok

Can't `copy` from a `byte` to a `word`.

    | byte source : 0
    | word dest
    | 
    | routine main
    |   inputs source
    |   outputs dest
    |   trashes a, z, n
    | {
    |     copy source, dest
    | }
    ? TypeMismatchError

Can't `copy` from a `word` to a `byte`.

    | word source : 0
    | byte dest
    | 
    | routine main
    |   inputs source
    |   outputs dest
    |   trashes a, z, n
    | {
    |     copy source, dest
    | }
    ? TypeMismatchError

### Buffers and pointers ###

Note that `^buf` is a constant value, so it by itself does not require `buf` to be
listed in any input/output sets.

However, if the code reads from it through a pointer, it *should* be in `inputs`.

Likewise, if the code writes to it through a pointer, it *should* be in `outputs`.

Of course, unless you write to *all* the bytes in a buffer, some of those bytes
might not be meaningful.  So how meaningful is this check?

This is an open problem.

For now, convention says: if it is being read, list it in `inputs`, and if it is
being modified, list it in both `inputs` and `outputs`.

Write literal through a pointer.

    | buffer[2048] buf
    | pointer ptr
    | 
    | routine main
    |   inputs buf
    |   outputs y, buf
    |   trashes a, z, n, ptr
    | {
    |     ld y, 0
    |     copy ^buf, ptr
    |     copy 123, [ptr] + y
    | }
    = ok

It does use `y`.

    | buffer[2048] buf
    | pointer ptr
    | 
    | routine main
    |   inputs buf
    |   outputs buf
    |   trashes a, z, n, ptr
    | {
    |     copy ^buf, ptr
    |     copy 123, [ptr] + y
    | }
    ? UnmeaningfulReadError

Write stored value through a pointer.

    | buffer[2048] buf
    | pointer ptr
    | byte foo
    | 
    | routine main
    |   inputs foo, buf
    |   outputs y, buf
    |   trashes a, z, n, ptr
    | {
    |     ld y, 0
    |     copy ^buf, ptr
    |     copy foo, [ptr] + y
    | }
    = ok

Read through a pointer.

    | buffer[2048] buf
    | pointer ptr
    | byte foo
    | 
    | routine main
    |   inputs buf
    |   outputs foo
    |   trashes a, y, z, n, ptr
    | {
    |     ld y, 0
    |     copy ^buf, ptr
    |     copy [ptr] + y, foo
    | }
    = ok

Read and write through two pointers.

    | buffer[2048] buf
    | pointer ptra
    | pointer ptrb
    | 
    | routine main
    |   inputs buf
    |   outputs buf
    |   trashes a, y, z, n, ptra, ptrb
    | {
    |     ld y, 0
    |     copy ^buf, ptra
    |     copy ^buf, ptrb
    |     copy [ptra] + y, [ptrb] + y
    | }
    = ok

Read through a pointer to the `a` register.  Note that this is done with `ld`,
not `copy`.

    | buffer[2048] buf
    | pointer ptr
    | byte foo
    | 
    | routine main
    |   inputs buf
    |   outputs a
    |   trashes y, z, n, ptr
    | {
    |     ld y, 0
    |     copy ^buf, ptr
    |     ld a, [ptr] + y
    | }
    = ok

Write the `a` register through a pointer.  Note that this is done with `st`,
not `copy`.

    | buffer[2048] buf
    | pointer ptr
    | byte foo
    | 
    | routine main
    |   inputs buf
    |   outputs buf
    |   trashes a, y, z, n, ptr
    | {
    |     ld y, 0
    |     copy ^buf, ptr
    |     ld a, 255
    |     st a, [ptr] + y
    | }
    = ok

### routines ###

Routines are constants.  You need not, and in fact cannot, specify a constant
as an input to, an output of, or as a trashed value of a routine.

    | vector routine
    |   inputs x
    |   outputs x
    |   trashes z, n
    |     vec
    | 
    | routine foo
    |   inputs x
    |   outputs x
    |   trashes z, n
    | {
    |   inc x
    | }
    | 
    | routine main
    |   inputs foo
    |   outputs vec
    |   trashes a, z, n
    | {
    |     copy foo, vec
    | }
    ? ConstantConstraintError: foo

    | vector routine
    |   inputs x
    |   outputs x
    |   trashes z, n
    |     vec
    | 
    | routine foo
    |   inputs x
    |   outputs x
    |   trashes z, n
    | {
    |   inc x
    | }
    | 
    | routine main
    |   outputs vec, foo
    |   trashes a, z, n
    | {
    |     copy foo, vec
    | }
    ? ConstantConstraintError: foo

    | vector routine
    |   inputs x
    |   outputs x
    |   trashes z, n
    |     vec
    | 
    | routine foo
    |   inputs x
    |   outputs x
    |   trashes z, n
    | {
    |   inc x
    | }
    | 
    | routine main
    |   outputs vec
    |   trashes a, z, n, foo
    | {
    |     copy foo, vec
    | }
    ? ConstantConstraintError: foo

#### routine-vector type compatibility

You can copy the address of a routine into a vector, if that vector type
is at least as "wide" as the type of the routine.  More specifically,

- the vector must take _at least_ the inputs that the routine takes
- the vector must produce _at least_ the outputs that the routine produces
- the vector must trash _at least_ what the routine trashes

If the vector and the routine have the very same signature, that's not an error.

    | vector routine
    |   inputs x, y
    |   outputs x, y
    |   trashes z, n
    |     vec
    | 
    | routine foo
    |   inputs x, y
    |   outputs x, y
    |   trashes z, n
    | {
    |   inc x
    |   inc y
    | }
    | 
    | routine main
    |   outputs vec
    |   trashes a, z, n
    | {
    |     copy foo, vec
    | }
    = ok

If the vector takes an input that the routine doesn't take, that's not an error.
(The interface requires that a parameter be specified before calling, but the
implementation doesn't actually read it.)

    | vector routine
    |   inputs x, y, a
    |   outputs x, y
    |   trashes z, n
    |     vec
    | 
    | routine foo
    |   inputs x, y
    |   outputs x, y
    |   trashes z, n
    | {
    |   inc x
    |   inc y
    | }
    | 
    | routine main
    |   outputs vec
    |   trashes a, z, n
    | {
    |     copy foo, vec
    | }
    = ok

If the vector fails to take an input that the routine takes, that's an error.

    | vector routine
    |   inputs x
    |   outputs x, y
    |   trashes z, n
    |     vec
    | 
    | routine foo
    |   inputs x, y
    |   outputs x, y
    |   trashes z, n
    | {
    |   inc x
    |   inc y
    | }
    | 
    | routine main
    |   outputs vec
    |   trashes a, z, n
    | {
    |     copy foo, vec
    | }
    ? IncompatibleConstraintsError

If the vector produces an output that the routine doesn't produce, that's not an error.
(The interface claims the result of calling the routine is defined, but the implementation
actually preserves it instead of changing it; the caller can still treat it as a defined
output.)

    | vector routine
    |   inputs x, y
    |   outputs x, y, a
    |   trashes z, n
    |     vec
    | 
    | routine foo
    |   inputs x, y
    |   outputs x, y
    |   trashes z, n
    | {
    |   inc x
    |   inc y
    | }
    | 
    | routine main
    |   outputs vec
    |   trashes a, z, n
    | {
    |     copy foo, vec
    | }
    = ok

If the vector fails to produce an output that the routine produces, that's an error.

    | vector routine
    |   inputs x, y
    |   outputs x
    |   trashes z, n
    |     vec
    | 
    | routine foo
    |   inputs x, y
    |   outputs x, y
    |   trashes z, n
    | {
    |   inc x
    |   inc y
    | }
    | 
    | routine main
    |   outputs vec
    |   trashes a, z, n
    | {
    |     copy foo, vec
    | }
    ? IncompatibleConstraintsError

If the vector fails to trash something the routine trashes, that's an error.

    | vector routine
    |   inputs x, y
    |   outputs x, y
    |   trashes z
    |     vec
    | 
    | routine foo
    |   inputs x, y
    |   outputs x, y
    |   trashes z, n
    | {
    |   inc x
    |   inc y
    | }
    | 
    | routine main
    |   outputs vec
    |   trashes a, z, n
    | {
    |     copy foo, vec
    | }
    ? IncompatibleConstraintsError

If the vector trashes something the routine doesn't trash, that's not an error.
(The implementation preserves something the interface doesn't guarantee is
preserved.  The caller gets no guarantee that it's preserved.  It actually is,
but it doesn't know that.)

    | vector routine
    |   inputs x, y
    |   outputs x, y
    |   trashes a, z, n
    |     vec
    | 
    | routine foo
    |   inputs x, y
    |   outputs x, y
    |   trashes z, n
    | {
    |   inc x
    |   inc y
    | }
    | 
    | routine main
    |   outputs vec
    |   trashes a, z, n
    | {
    |     copy foo, vec
    | }
    = ok

#### other properties of routines

Routines are read-only.

    | vector routine
    |   inputs x
    |   outputs x
    |   trashes z, n
    |     vec
    | 
    | routine foo
    |   inputs x
    |   outputs x
    |   trashes z, n
    | {
    |   inc x
    | }
    | 
    | routine main
    |   outputs vec
    |   trashes a, z, n
    | {
    |     copy vec, foo
    | }
    ? TypeMismatchError

Indirect call.

    | vector routine
    |   outputs x trashes z, n
    |     foo
    | 
    | routine bar outputs x trashes z, n {
    |     ld x, 200
    | }
    | 
    | routine main outputs x, foo trashes a, z, n {
    |     copy bar, foo
    |     call foo
    | }
    = ok

Calling the vector does indeed trash the things the vector says it does.

    | vector routine trashes x, z, n foo
    | 
    | routine bar trashes x, z, n {
    |     ld x, 200
    | }
    | 
    | routine main outputs x, foo trashes z, n {
    |     ld x, 0
    |     copy bar, foo
    |     call foo
    | }
    ? UnmeaningfulOutputError: x

`goto`, if present, must be in tail position (the final instruction in a routine.)

    | routine bar trashes x, z, n {
    |     ld x, 200
    | }
    | 
    | routine main trashes x, z, n {
    |     ld x, 0
    |     goto bar
    | }
    = ok

    | routine bar trashes x, z, n {
    |     ld x, 200
    | }
    | 
    | routine main trashes x, z, n {
    |     goto bar
    |     ld x, 0
    | }
    ? IllegalJumpError

    | routine bar trashes x, z, n {
    |     ld x, 200
    | }
    | 
    | routine main trashes x, z, n {
    |     ld x, 0
    |     if z {
    |         ld x, 1
    |         goto bar
    |     }
    | }
    = ok

    | routine bar trashes x, z, n {
    |     ld x, 200
    | }
    | 
    | routine main trashes x, z, n {
    |     ld x, 0
    |     if z {
    |         ld x, 1
    |         goto bar
    |     }
    |     ld x, 0
    | }
    ? IllegalJumpError

    | routine bar trashes x, z, n {
    |     ld x, 200
    | }
    | 
    | routine main trashes x, z, n {
    |     ld x, 0
    |     if z {
    |         ld x, 1
    |         goto bar
    |     } else {
    |         ld x, 0
    |         goto bar
    |     }
    | }
    = ok

    | routine bar trashes x, z, n {
    |     ld x, 200
    | }
    | 
    | routine main trashes x, z, n {
    |     ld x, 0
    |     if z {
    |         ld x, 1
    |         goto bar
    |     } else {
    |         ld x, 0
    |     }
    | }
    = ok

For the purposes of `goto`, the end of a loop is never tail position.

    | routine bar trashes x, z, n {
    |     ld x, 200
    | }
    | 
    | routine main trashes x, z, n {
    |     ld x, 0
    |     repeat {
    |         inc x
    |         goto bar
    |     } until z
    | }
    ? IllegalJumpError

Can't `goto` a routine that outputs or trashes more than the current routine.

    | routine bar trashes x, y, z, n {
    |     ld x, 200
    |     ld y, 200
    | }
    | 
    | routine main trashes x, z, n {
    |     ld x, 0
    |     goto bar
    | }
    ? IncompatibleConstraintsError

    | routine bar outputs y trashes z, n {
    |     ld y, 200
    | }
    | 
    | routine main trashes x, z, n {
    |     ld x, 0
    |     goto bar
    | }
    ? IncompatibleConstraintsError

Can `goto` a routine that outputs or trashes less than the current routine.

    | routine bar trashes x, z, n {
    |     ld x, 1
    | }
    | 
    | routine main trashes a, x, z, n {
    |     ld a, 0
    |     ld x, 0
    |     goto bar
    | }
    = ok

Indirect goto.

    | vector routine outputs x trashes a, z, n foo
    | 
    | routine bar outputs x trashes a, z, n {
    |     ld x, 200
    | }
    | 
    | routine main outputs x trashes foo, a, z, n {
    |     copy bar, foo
    |     goto foo
    | }
    = ok

Jumping through the vector does indeed trash, or output, the things the
vector says it does.

    | vector routine
    |   trashes a, x, z, n
    |     foo
    | 
    | routine bar
    |   trashes a, x, z, n {
    |     ld x, 200
    | }
    | 
    | routine sub
    |   trashes foo, a, x, z, n {
    |     ld x, 0
    |     copy bar, foo
    |     goto foo
    | }
    | 
    | routine main
    |   outputs a
    |   trashes foo, x, z, n {
    |     call sub
    |     ld a, x
    | }
    ? UnmeaningfulReadError: x

    | vector routine
    |   outputs x
    |   trashes a, z, n  foo
    | 
    | routine bar
    |   outputs x
    |   trashes a, z, n {
    |     ld x, 200
    | }
    | 
    | routine sub
    |   outputs x
    |   trashes foo, a, z, n {
    |     ld x, 0
    |     copy bar, foo
    |     goto foo
    | }
    | 
    | routine main
    |   outputs a
    |   trashes foo, x, z, n {
    |     call sub
    |     ld a, x
    | }
    = ok

### vector tables ###

A vector can be copied into a vector table.

    | vector routine
    |   outputs x
    |   trashes a, z, n
    |     one
    | vector (routine
    |   outputs x
    |   trashes a, z, n)
    |     table[256] many
    | 
    | routine bar outputs x trashes a, z, n {
    |     ld x, 200
    | }
    | 
    | routine main
    |   inputs one, many
    |   outputs one, many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     copy bar, one
    |     copy one, many + x
    | }
    = ok

A vector can be copied out of a vector table.

    | vector routine
    |   outputs x
    |   trashes a, z, n
    |     one
    | vector (routine
    |   outputs x
    |   trashes a, z, n)
    |     table[256] many
    | 
    | routine bar outputs x trashes a, z, n {
    |     ld x, 200
    | }
    | 
    | routine main
    |   inputs one, many
    |   outputs one, many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     copy many + x, one
    |     call one
    | }
    = ok

A routine can be copied into a vector table.

    | vector (routine
    |     outputs x
    |     trashes a, z, n)
    |   table[256] many
    | 
    | routine bar outputs x trashes a, z, n {
    |     ld x, 200
    | }
    | 
    | routine main
    |   inputs many
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     copy bar, many + x
    | }
    = ok

A vector in a vector table cannot be directly called.

    | vector (routine
    |     outputs x
    |     trashes a, z, n)
    |   table[256] many
    | 
    | routine bar outputs x trashes a, z, n {
    |     ld x, 200
    | }
    | 
    | routine main
    |   inputs many
    |   outputs many
    |   trashes a, x, n, z
    | {
    |     ld x, 0
    |     copy bar, many + x
    |     call many + x
    | }
    ? SyntaxError

### typedef ###

A typedef is a more-readable alias for a type.  "Alias" means
that types have structural equivalence, not name equivalence.

    | typedef routine
    |   inputs x
    |   outputs x
    |   trashes z, n
    |     routine_type
    | 
    | vector routine_type vec
    | 
    | routine foo
    |   inputs x
    |   outputs x
    |   trashes z, n
    | {
    |   inc x
    | }
    | 
    | routine main
    |   outputs vec
    |   trashes a, z, n
    | {
    |     copy foo, vec
    | }
    = ok

The new style routine definitions support typedefs.

    | typedef routine
    |   inputs x
    |   outputs x
    |   trashes z, n
    |     routine_type
    | 
    | vector routine_type vec
    | 
    | define foo routine_type
    | {
    |   inc x
    | }
    | 
    | routine main
    |   outputs vec
    |   trashes a, z, n
    | {
    |     copy foo, vec
    | }
    = ok

### static ###

When memory locations are defined static to a routine, they cannot be
directly input, nor directly output; and since they are always initialized,
they cannot be trashed.  Thus, they really don't participate in the analysis.

    | define foo routine
    |   inputs x
    |   outputs x
    |   trashes z, n
    |   static byte t : 0
    | {
    |   st x, t
    |   inc t
    |   ld x, t
    | }
    | 
    | define main routine
    |   trashes a, x, z, n
    |   static byte t : 0
    | {
    |   ld x, t
    |   call foo
    | }
    = ok