Rudimentarily document for; add failing test cases for it.

doc/SixtyPical.md

@@ -531,6 +531,22 @@ The sense of the test can be inverted with `not`.
         }
     } until not z
 
+### for ###
+
+    for <dest-memory-location> (up|down) to <literal-byte> {
+        <block>
+    }
+
+Executes the block repeatedly, incrementing or decrementing the
+dest-memory-location at the end of the block, until the value of
+the dest-memory-location has gone past the literal-byte.
+
+The block is always executed as least once.
+
+*   It is illegal if any memory location is uninitialized at the exit of
+    the loop when that memory location is initialized at the start of
+    the loop.
+
 Grammar
 -------
 

tests/SixtyPical Analysis.md

@@ -1755,6 +1755,40 @@ If the range isn't known to be larger than the final value, you can't go down to
     | }
     ? 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
+
 ### copy ###
 
 Can't `copy` from a memory location that isn't initialized.