(defprotocol PStack
  "A stack protocol"
  (push [this val] "Push an element onto the stack")
  (mypop [this] "Pop an element from the top of the stack")
  (top [this] "Return the element from the top of the stack"))

(deftype Stack [t h]
  PStack
  (push [this val]
        (Stack. this val))
  (mypop [_] t)
  (top [_] h))

(deftype EmptyStack []
  PStack
  (push [this val]
        (Stack. this val))
  (mypop [_] (throw (IllegalStateException. "Can't pop an empty stack")))
  (top [_] nil))