diff --git a/src/sixtypical/parser.py b/src/sixtypical/parser.py
index 5007d53..618ea2a 100644
--- a/src/sixtypical/parser.py
+++ b/src/sixtypical/parser.py
@@ -29,6 +29,8 @@ class Parser(object):
             raise SyntaxError('Undefined symbol "%s"' % name)
         return self.symbols[name].model
 
+    # --- grammar productions
+
     def program(self):
         defns = []
         routines = []
@@ -47,6 +49,19 @@ class Parser(object):
             self.symbols[name] = SymEntry(routine, routine.location)
             routines.append(routine)
         self.scanner.check_type('EOF')
+        # now backpatch the executable types.
+        for defn in defns:
+            if isinstance(defn.location.type, VectorType):
+                t = defn.location.type
+                t.inputs = set([self.lookup(w) for w in t.inputs])
+                t.outputs = set([self.lookup(w) for w in t.outputs])
+                t.trashes = set([self.lookup(w) for w in t.trashes])
+        for routine in routines:
+            if isinstance(routine.location.type, ExecutableType):
+                t = routine.location.type
+                t.inputs = set([self.lookup(w) for w in t.inputs])
+                t.outputs = set([self.lookup(w) for w in t.outputs])
+                t.trashes = set([self.lookup(w) for w in t.trashes])
         return Program(defns=defns, routines=routines)
 
     def defn(self):
@@ -108,11 +123,11 @@ class Parser(object):
         outputs = set()
         trashes = set()
         if self.scanner.consume('inputs'):
-            inputs = set(self.locexprs())
+            inputs = set(self.labels())
         if self.scanner.consume('outputs'):
-            outputs = set(self.locexprs())
+            outputs = set(self.labels())
         if self.scanner.consume('trashes'):
-            trashes = set(self.locexprs())
+            trashes = set(self.labels())
         return (inputs, outputs, trashes)
 
     def routine(self):
@@ -137,6 +152,20 @@ class Parser(object):
             location=location
         )
 
+    def labels(self):
+        accum = []
+        accum.append(self.label())
+        while self.scanner.consume(','):
+            accum.append(self.label())
+        return accum
+
+    def label(self):
+        """Like a locexpr, but does not allow literal values, and the labels do not
+        need to be defined yet.  They will be resolved at the end of parsing."""
+        loc = self.scanner.token
+        self.scanner.scan()
+        return loc
+
     def locexprs(self):
         accum = []
         accum.append(self.locexpr())
diff --git a/tests/SixtyPical Syntax.md b/tests/SixtyPical Syntax.md
index b7b6c73..8fdebee 100644
--- a/tests/SixtyPical Syntax.md	
+++ b/tests/SixtyPical Syntax.md	
@@ -305,6 +305,25 @@ Only vectors can be decorated with constraints like that.
     | }
     ? SyntaxError
 
+Constraints set may only contain labels.
+
+    | vector cinv
+    |   inputs a
+    |   outputs 200
+    |   trashes a, x, z, n
+    |   @ 788
+    | 
+    | routine foo {
+    |     ld a, 0
+    | }
+    | routine main {
+    |     with interrupts off {
+    |         copy foo, cinv
+    |     }
+    |     call cinv
+    | }
+    ? SyntaxError
+
 A vector can name itself in its inputs, outputs, and trashes.
 
     | vector cinv