Confirm constraint on saveing a.

src/sixtypical/analyzer.py

@@ -785,7 +785,8 @@ class Analyzer(object):
             baton = batons.pop()
             context.re_introduce(baton)
 
-        # FIXME check if A needs to be the outer thing that is saved, I think it does.
+        # We do this check outside the loop, because A is only preserved
+        # if it is the outermost thing being `save`d.
         if location == REG_A:
             pass
         else:

tests/SixtyPical Analysis.md

@@ -2121,6 +2121,20 @@ There is a shortcut syntax for a nested series of `save`s.
     | }
     = ok
 
+`a` is only preserved if it is the outermost thing `save`d.
+
+    | routine main
+    |   inputs a
+    |   outputs a
+    |   trashes z, n
+    | {
+    |     save x, a {
+    |         ld a, 0
+    |         ld x, 1
+    |     }
+    | }
+    ? UnmeaningfulOutputError: a
+
 Not just registers, but also user-defined locations can be saved.
 
     | byte foo