Derive scalar changes and composition laws

test/test_harness.mli

@@ -21,3 +21,11 @@ val parse_error : string -> Diagnostic.t option
 val resolve_error : string -> Diagnostic.t option
 val infer_error : string -> Diagnostic.t option
 val check_message : string -> string -> (unit -> 'a) -> unit
+
+type rng = { mutable state : int }
+
+val rng : int -> rng
+val next : rng -> int
+val range : rng -> int -> int
+val pick : rng -> 'a list -> 'a
+val string_of_int_list : int list -> string