Assert that the loop variable is meaningful in repeat. Unit test.

src/sixtypical/analyzer.py

@@ -286,12 +286,13 @@ class Analyzer(object):
             # it will always be executed at least once, so analyze it having
             # been executed the first time.
             self.analyze_block(instr.block, context)
-    
+            context.assert_meaningful(src)
+
             # now analyze it having been executed a second time, with the context
             # of it having already been executed.
             self.analyze_block(instr.block, context)
-    
-            # NB I *think* that's enough... but it might not be?
+            context.assert_meaningful(src)
+
         elif opcode == 'copy':
             # check that their types are basically compatible
             if src.type == dest.type:

tests/SixtyPical Analysis.md

@@ -1017,6 +1017,23 @@ initialized at the start.
     | }
     ? UnmeaningfulReadError: y in main
 
+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 in main
+
 ### copy ###
 
 Can't `copy` from a memory location that isn't initialized.