Saving anything except a trashes a.

src/sixtypical/analyzer.py

@@ -777,3 +777,9 @@ class Analyzer(object):
         self.analyze_block(instr.block, context)
         # TODO assert no goto was encountered
         context.re_introduce(baton)
+
+        if location == REG_A:
+            pass
+        else:
+            context.set_touched(REG_A)
+            context.set_unmeaningful(REG_A)

tests/SixtyPical Analysis.md

@@ -1955,17 +1955,45 @@ Basic neutral test, where the `save` makes no difference.
     | }
     = 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
-    |   inputs a
+    |   outputs x, y
-    |   outputs a, x
+    |   trashes a, z, n
-    |   trashes z, n
     | {
     |     ld x, 0
     |     save x {
-    |         ld a, 0
+    |         ld y, 0
     |         trash x
     |     }
     | }
@@ -1991,16 +2019,32 @@ It will continue to be trashed outside the block.
 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
 
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