Compose lenses whilst preserving unfocussed data

runtime.ml

@@ -13,3 +13,10 @@ let () =
   let calls = ref 0 in
   assert (over sample ~f:(fun text -> incr calls; text ^ "s") source = ("colours", true));
   assert (!calls = 1)
+
+let () =
+  let first = (fst, fun value (_, rest) -> value, rest) in
+  let second = (snd, fun value (rest, _) -> rest, value) in
+  let focus = compose first second in
+  assert (view focus ((true, "grey"), false) = "grey");
+  assert (set focus 9 ((true, "grey"), false) = ((true, 9), false))