type 'a queue = Q of 'a list * 'a list ;;

let empty = Q([],[]) ;;

let is_empty q = (q = empty) ;;

let add q a = if is_empty q then Q([a],[])
	                 else match q with Q(l1,l2) -> Q(l1,l2@[a]);;

exception Empty;;

let peek q = if is_empty q then failwith "????"
	                   else match q with Q([],h::t) -> h
					   | Q(h::t,l2) -> h;;

let pop q = if is_empty q then failwith "????"
	                  else match q with Q([],h::t) -> Q([],t)
					  | Q(h::t,l2) -> Q(t,l2);;

let map f q = match q with