diff --git a/src/test-lib/test_util.ml b/src/test-lib/test_util.ml
new file mode 100644
index 0000000..d2a822e
--- /dev/null
+++ b/src/test-lib/test_util.ml
@@ -0,0 +1,10 @@
+
+(** Check if two lists contain the same items with the same multiplicities,
+    but in any order. Consider items equal if [eq x y] (default: [x = y]) holds. *)
+let rec equal_unordered ?(eq = (=)) xs ys =
+  match xs with
+  | [] -> ys = []
+  | x :: xs' ->
+    match List.partition (eq x) ys with
+    | [], _ -> false
+    | _ :: ys1', ys2' -> equal_unordered ~eq xs' (ys1' @ ys2')