diff --git a/basis/compiler/tree/propagation/branches/branches.factor b/basis/compiler/tree/propagation/branches/branches.factor
index 28f34cb425..ef9e4e8f0b 100644
--- a/basis/compiler/tree/propagation/branches/branches.factor
+++ b/basis/compiler/tree/propagation/branches/branches.factor
@@ -1,8 +1,8 @@
-! Copyright (C) 2008, 2009 Slava Pestov.
+! Copyright (C) 2008, 2010 Slava Pestov.
 ! See http://factorcode.org/license.txt for BSD license.
-USING: fry kernel sequences assocs accessors namespaces
+USING: fry kernel sequences assocs accessors
 math.intervals arrays classes.algebra combinators columns
-stack-checker.branches locals math
+stack-checker.branches locals math namespaces
 compiler.utilities
 compiler.tree
 compiler.tree.combinators
@@ -10,6 +10,8 @@ compiler.tree.propagation.info
 compiler.tree.propagation.nodes
 compiler.tree.propagation.simple
 compiler.tree.propagation.constraints ;
+FROM: sets => union ;
+FROM: assocs => change-at ;
 IN: compiler.tree.propagation.branches
 
 ! For conditionals, an assoc of child node # --> constraint
@@ -90,7 +92,7 @@ M: #phi propagate-before ( #phi -- )
     bi ;
 
 :: update-constraints ( new old -- )
-    new [| key value | key old [ value append ] change-at ] assoc-each ;
+    new [| key value | key old [ value union ] change-at ] assoc-each ;
 
 : include-child-constraints ( i -- )
     infer-children-data get nth constraints swap at last
diff --git a/basis/compiler/tree/propagation/constraints/constraints.factor b/basis/compiler/tree/propagation/constraints/constraints.factor
index 617352d699..f9988ba220 100644
--- a/basis/compiler/tree/propagation/constraints/constraints.factor
+++ b/basis/compiler/tree/propagation/constraints/constraints.factor
@@ -1,4 +1,4 @@
-! Copyright (C) 2008 Slava Pestov.
+! Copyright (C) 2008, 2010 Slava Pestov.
 ! See http://factorcode.org/license.txt for BSD license.
 USING: arrays assocs math math.intervals kernel accessors
 sequences namespaces classes classes.algebra
@@ -87,8 +87,11 @@ TUPLE: implication p q ;
 
 C: --> implication
 
+: maybe-add ( elt seq -- seq' )
+    2dup member? [ nip ] [ swap suffix ] if ;
+
 : assume-implication ( q p -- )
-    [ constraints get [ assoc-stack swap suffix ] 2keep last set-at ]
+    [ constraints get [ assoc-stack maybe-add ] 2keep last set-at ]
     [ satisfied? [ assume ] [ drop ] if ] 2bi ;
 
 M: implication assume*