(* Exchange the first elements of two lists *)
let exchangeFirst (l1, l2) =
  match l1 with
      [] -> (l1, l2)
    | head1::tail1 ->
      (match l2 with
	  [] -> (l1, l2)
	| head2::tail2 -> (head2::tail1, head1::tail2))

let exchangeFirst2 (l1, l2) =
  match (l1, l2) with
      (_, []) | ([], _) -> (l1, l2)
    | ((h1:int)::t1, h2::t2) -> (h2::t1, h1::t2)

(* Just some polymorphic type fun *)
type 'a ok =
    OKInt of int
  | OKString of string
  | OKSomething of 'a

(* doing something with boolean whatevers *)
type boolsomething =
    True
  | False
  | Not of boolsomething
  | Or of boolsomething * boolsomething
  | And of boolsomething * boolsomething

exception Not_correct
let rec do_something_boolean something =
  match something with
      True -> True
    | False -> False
    | Not q ->
      (match do_something_boolean q with
	  True -> False
	| False -> True
	| _ -> raise Not_correct) (* WILL NEVER HAPPEN *)
    | Or (p, q) ->
      let p2 = do_something_boolean p in
      let q2 = do_something_boolean q in
      (match (p2, q2) with
	  (_, True) | (True, _) -> True
	| _ -> False)
    | And (p, q) ->
      (match (do_something_boolean p, do_something_boolean q) with
	  (_, False) | (False, _) -> False
	| _ -> True)

(* detect even and odd numbers *)
let rec even n =
  if n = 0
  then true
  else odd (n - 1)
and odd n =
  if n = 0
  then false
  else even (n - 1)