import List;

fun emptyQueue () {
  [{}, {}]
}

fun isEmptyQueue (q) {
  case q of
    [{}, {}] -> true
  | _        -> false
  esac
}

fun enqueue ([pop, push], x) {
  [pop, x : push]
}

fun dequeue ([pop, push]) {
  case pop of
    x : pop -> [[pop, push], x]
  | _       -> case push of
                 {} -> failure ("dequeueing from empty queue\n")
               | _  -> dequeue ([reverse (push), {}])
	       esac
  esac
}