module Stack = struct
  type 'a stack = { mutable c : 'a list }
  exception EmptyStackException
 
  let empty = { c = [] }

  let push s x = s.c <- x :: s.c
 
  let pop s =
    match s.c with
      hd::tl -> s.c <- tl
    | []     -> raise EmptyStackException
 
  let top s =
    match s.c with
      hd::_ -> hd
    | []    -> raise EmptyStackException
 
  let is_empty s = (s.c = [])
end;;