Provide the identity lens and verify composition identities

runtime.ml

@@ -25,3 +25,8 @@ let () =
   assert (set _1 "grey" (1, true) = ("grey", true));
   assert (over _2 ~f:string_of_int (false, 9) = (false, "9"));
   assert (view (compose _1 _2) ((true, "grey"), ()) = "grey")
+
+let () =
+  assert (set _id "grey" 9 = "grey");
+  assert (view (compose _id _1) (9, false) = view _1 (9, false));
+  assert (set (compose _2 _id) "grey" (true, 9) = set _2 "grey" (true, 9))