diff --git a/play.ml b/play.ml
index 6ab4e7758e9f3a49f44c32ca43841c31935ec2a8..84e995617c6df7074aad6a6e079f9f1d4f3c40e6 100644
--- a/play.ml
+++ b/play.ml
@@ -48,9 +48,14 @@ let print_int_piter =
 #install_printer print_int_piter;;
 
 (* Using Sek directly, with default settings. *)
+let sum e = E.fold_left (+) 0 e;;
 let e = E.of_array (-1) [|0;1;2;3|];;
-let sum = E.fold_left (+) 0 e;;
-assert (sum = 6);;
+assert (sum e = 6);;
+let e' = E.copy e;;
+E.set e' 0 42;;
+assert (sum e = 6);;
+assert (sum e' = 48);;
+
 let p = P.of_array (-1) (Array.init 32 (fun i -> i));;
 let sum = P.fold_left (+) 0 p;;
 assert (sum = 496);;