1 files changed, 3 insertions, 2 deletions
diff --git a/tree.sig b/tree.sig
index afab259..8caf4cc 100644
--- a/tree.sig
+++ b/tree.sig
@@ -1,10 +1,11 @@
 signature TREE = sig
   type ('k, 'v) t
-  exception Exists
 
   val empty: ('k, 'v) t
 
-  val insert: ('k -> 'k -> order) -> ('k, 'v) t -> 'k -> 'v -> ('k, 'v) t
+  val insert: ('k -> 'k -> order) -> ('k, 'v) t -> 'k -> 'v
+        -> 'v option * ('k, 'v) t
+
   val lookup: ('k -> 'k -> order) -> ('k, 'v) t -> 'k -> 'v option
   val lookup2: ('k -> 'k -> order) -> ('k, 'v) t -> 'k ->
         ('v -> 'v option * 'a) * 'a -> 'a * ('k, 'v) t