# Return a FIFO proxy queue where `result.take` is `shift`.
	#
	# The point of such a proxy is to let the user choose the underling data structure
	# behind the FIFO.
	#
	# Note that some `Sequence`, like `Array`, have an inefficient `shift` implementation,
	# thus are not the best candidates to serve as a FIFO.
	#
	# ~~~
	# var a = new List[Int].from([1,2,3])
	# var q = a.as_fifo
	# assert q.take == 1
	# q.add(4)
	# q.add(5)
	# assert q.take == 2
	# assert a == [3, 4, 5]
	# ~~~
	fun as_fifo: Queue[E] do return new FifoQueue[E](self)