diff --git a/src/Data/Syntax/Assignment.hs b/src/Data/Syntax/Assignment.hs
index f65e91f7b..b317bc718 100644
--- a/src/Data/Syntax/Assignment.hs
+++ b/src/Data/Syntax/Assignment.hs
@@ -147,6 +147,7 @@ instance Enum symbol => Alternative (Assignment (Node symbol)) where
     (Symbol s1 `Then` _, Symbol s2 `Then` _) -> Choose (IntMap.fromListWith (flip const) [(fromEnum s1, a), (fromEnum s2, b)]) `Then` identity
     (Symbol s `Then` _, Choose choices `Then` continue) -> Choose (IntMap.insertWith const (fromEnum s) a (fmap continue choices)) `Then` identity
     (Choose choices `Then` continue, Symbol s `Then` _) -> Choose (IntMap.insertWith (flip const) (fromEnum s) b (fmap continue choices)) `Then` identity
+    (Choose choices1 `Then` continue1, Choose choices2 `Then` continue2) -> Choose (IntMap.union (fmap continue1 choices1) (fmap continue2 choices2)) `Then` identity
     _ -> wrap $ Alt a b
 
 instance Show symbol => Show1 (AssignmentF (Node symbol)) where