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signature Queue =
sig
  type a Queue

val empty   : a Queue
val isEmpty : a Queue -> bool

val snoc    : a Queue × a -> a Queue
val head    : a Queue -> a       (* raises Empty if queue is empty *)
val tail    : a Queue -> a Queue (* raises Empty if queue is empty *)

type a Queue = a list × a list

type a Queue = int × a Stream × int × a Stream

type a Queue = a list × int × a list susp × int × a list

type a Queue = a Stream × a list × a Stream

datatype a RotationState =
      Idle
    | Reversing of int × a list × a list × a list × a list
    | Appending of int × a list × a list
    | Done of a list

type a Queue = int × a list × a RotationState × int × a list