diff --git a/README.md b/README.md
index 34ddc1d..f55f6ef 100644
--- a/README.md
+++ b/README.md
@@ -31,7 +31,6 @@ Documentation
 *   [SixtyPical specification](doc/SixtyPical.md)
 *   [SixtyPical revision history](HISTORY.md)
 *   [Literate test suite for SixtyPical syntax](tests/SixtyPical%20Syntax.md)
-*   [Literate test suite for SixtyPical execution](tests/SixtyPical%20Execution.md)
 *   [Literate test suite for SixtyPical analysis](tests/SixtyPical%20Analysis.md)
 *   [Literate test suite for SixtyPical compilation](tests/SixtyPical%20Compilation.md)
 *   [6502 Opcodes used/not used in SixtyPical](doc/6502%20Opcodes.md)
@@ -78,11 +77,14 @@ are trashed inside the block.
 ### Re-order routines and optimize tail-calls to fallthroughs
 
 Not because it saves 3 bytes, but because it's a neat trick.  Doing it optimally
-is probably NP-complete.  But doing it adeuqately is probably not that hard.
+is probably NP-complete.  But doing it adequately is probably not that hard.
+
+### Different preludes for different architectures
+
+`--prelude=c64-basic`
 
 ### And at some point...
 
-*   Confirm that `and` can be used to restrict the range of table reads/writes.
 *   `low` and `high` address operators - to turn `word` type into `byte`.
 *   `const`s that can be used in defining the size of tables, etc.
 *   Tests, and implementation, ensuring a routine can be assigned to a vector of "wider" type
diff --git a/src/sixtypical/analyzer.py b/src/sixtypical/analyzer.py
index d3b977b..88088aa 100644
--- a/src/sixtypical/analyzer.py
+++ b/src/sixtypical/analyzer.py
@@ -194,6 +194,7 @@ class Context(object):
     def set_touched(self, *refs):
         for ref in refs:
             self._touched.add(ref)
+            # TODO: it might be possible to invalidate the range here
 
     def set_meaningful(self, *refs):
         for ref in refs:
@@ -220,6 +221,10 @@ class Context(object):
             src_range = src.max_range()
         self._range[dest] = src_range
 
+    def invalidate_range(self, ref):
+        self.assert_meaningful(ref)
+        self._range[ref] = ref.max_range()
+
     def set_unmeaningful(self, *refs):
         for ref in refs:
             if ref in self._range:
@@ -377,6 +382,7 @@ class Analyzer(object):
                 raise TypeMismatchError(instr, '{} and {}'.format(src, name))
             else:
                 context.set_written(dest)
+                # FIXME: context.copy_range(src, dest)   ?
             context.assert_meaningful(src)
         elif opcode == 'add':
             context.assert_meaningful(src, dest, FLAG_C)
@@ -395,6 +401,7 @@ class Analyzer(object):
                     context.set_unmeaningful(REG_A)
                 else:
                     self.assert_type(TYPE_WORD, dest)
+            context.invalidate_range(dest)
         elif opcode == 'sub':
             context.assert_meaningful(src, dest, FLAG_C)
             if src.type == TYPE_BYTE:
@@ -405,10 +412,12 @@ class Analyzer(object):
                 context.set_written(dest, FLAG_Z, FLAG_N, FLAG_C, FLAG_V)
                 context.set_touched(REG_A)
                 context.set_unmeaningful(REG_A)
+            context.invalidate_range(dest)
         elif opcode in ('inc', 'dec'):
             self.assert_type(TYPE_BYTE, dest)
             context.assert_meaningful(dest)
             context.set_written(dest, FLAG_Z, FLAG_N)
+            context.invalidate_range(dest)
         elif opcode == 'cmp':
             self.assert_type(TYPE_BYTE, src, dest)
             context.assert_meaningful(src, dest)
@@ -425,10 +434,12 @@ class Analyzer(object):
             self.assert_type(TYPE_BYTE, src, dest)
             context.assert_meaningful(src, dest)
             context.set_written(dest, FLAG_Z, FLAG_N)
+            context.invalidate_range(dest)
         elif opcode in ('shl', 'shr'):
             self.assert_type(TYPE_BYTE, dest)
             context.assert_meaningful(dest, FLAG_C)
             context.set_written(dest, FLAG_Z, FLAG_N, FLAG_C)
+            context.invalidate_range(dest)
         elif opcode == 'call':
             type = instr.location.type
             if isinstance(type, VectorType):
diff --git a/tests/SixtyPical Analysis.md b/tests/SixtyPical Analysis.md
index 42d2173..47a9124 100644
--- a/tests/SixtyPical Analysis.md	
+++ b/tests/SixtyPical Analysis.md	
@@ -623,6 +623,62 @@ This applies to `copy` as well.
     | }
     ? 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.