type expr = Plus of expr * expr        (* means a + b *)
          | Minus of expr * expr       (* means a - b *)
          | Times of expr * expr       (* means a * b *)
	  | Divide of expr * expr      (* means a / b *)
          | Value of string            (* "x", "y", "n", etc. *)
	  ;;


let rec to_string e =
  match e with
    Plus (left, right)   -> "(" ^ (to_string left) ^ " + " ^ (to_string right) ^ ")"
  | Minus (left, right)  -> "(" ^ (to_string left) ^ " - " ^ (to_string right) ^ ")"
  | Times (left, right)  -> "(" ^ (to_string left) ^ " * " ^ (to_string right) ^ ")"
  | Divide (left, right) -> "(" ^ (to_string left) ^ " / " ^ (to_string right) ^ ")"
  | Value v -> v
  ;;

let print_expr e =
  print_endline (to_string e);;