1 files changed, 5 insertions, 0 deletions
diff --git a/libs/util.ml b/libs/util.ml
index cfa1619..522c3ec 100644
--- a/libs/util.ml
+++ b/libs/util.ml
@@ -1,6 +1,11 @@
 exception TypeError
 
 module VarMap = Mapext.Make(String)
+exception Duplicate of string
+let disjoint_union k a b = match a, b with
+  | Some x, None -> Some x
+  | None, Some y -> Some y 
+  | _ -> raise (Duplicate k)
 
 let rec fix equal f s =
   let fs = f s in