Make tests pass. Add two more tests towards completeness of spec.

src/sixtypical/analyzer.py

@@ -264,9 +264,15 @@ class Context(object):
         self.set_meaningful(*refs)
 
     def set_unwriteable(self, *refs):
+        """Intended to be used for implementing analyzing `for`."""
         for ref in refs:
             self._writeable.remove(ref)
 
+    def set_writeable(self, *refs):
+        """Intended to be used for implementing analyzing `for`."""
+        for ref in refs:
+            self._writeable.add(ref)
+
     def set_encountered_goto(self):
         self._has_encountered_goto = True
 
@@ -670,9 +676,18 @@ class Analyzer(object):
                 ))
             bottom = final
 
-        subcontext = context.clone()
-        subcontext.set_range(instr.dest, bottom, top)
-        subcontext.set_unwriteable(instr.dest)
-        self.analyze_block(instr.block, subcontext)
+        # inside the block, the loop variable cannot be modified, and we know its range.
+        context.set_range(instr.dest, bottom, top)
+        context.set_unwriteable(instr.dest)
 
+        # it will always be executed at least once, so analyze it having
+        # been executed the first time.
+        self.analyze_block(instr.block, context)
+
+        # now analyze it having been executed a second time, with the context
+        # of it having already been executed.
+        self.analyze_block(instr.block, context)
+
+        # after it is executed, we know the range of the loop variable.
         context.set_range(instr.dest, instr.final, instr.final)
+        context.set_writeable(instr.dest)

tests/SixtyPical Analysis.md

@@ -1685,6 +1685,17 @@ In a "for" loop, we know the exact range the loop variable takes on.
     | }
     ? 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
@@ -1731,6 +1742,17 @@ In a "for" loop (downward-counting variant), we know the exact range the loop va
     | }
     ? 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