diff --git a/theories/list.why b/theories/list.why
index ea6ab78d0869163e9e5c4cb9af769efbc8549037..8b2271cee57eea6f4fe0850542e1cffb5432806a 100644
--- a/theories/list.why
+++ b/theories/list.why
@@ -486,6 +486,18 @@ theory Prefix
 
 end
 
+theory Sum
+
+  use export List
+  use import int.Int
+
+  function sum (l: list int) : int = match l with
+  | Nil -> 0
+  | Cons x r -> x + sum r
+  end
+
+end
+
 (** {2 Induction on lists} *)
 
 theory Induction