# O(1)
	redef fun shift
	do
		var node = _head.as(not null)
		_head = node.next
		node.next = null
		if _head == null then
			_tail = null
		else
			_head.as(not null).prev = null
		end
		length -= 1
		return node.item
	end

	redef fun shift do
		var l = length - 1
		assert l >= 0
		length = l

		var native = _native
		var h = head
		var res = native[h]

		h += 1
		var cap = native.length
		if h >= cap then h -= cap
		head = h

		return res
	end

	redef fun shift
	do
		assert not_empty

		var node = head_node
		while node.length == 0 do
			# Delete empty node
			var nullable_node = node.next
			assert is_not_empty: nullable_node != null
			node = nullable_node
			node.prev = null
			self.head_node = node
		end

		var item = node.items[node.head_index]
		node.head_index += 1
		length -= 1
		return item
	end

	redef fun shift do
		var e = entities.shift
		e.set_parent null
		return e
	end

	redef fun shift
	do
		assert not_empty: not is_empty
		var r = first
		var l = length-1
		copy_to(1, l, self, 0)
		_length = l
		return r
	end

	redef fun shift
	do
		mutex.lock
		var r = real_collection.shift
		mutex.unlock
		return r
	end

	redef fun shift
	do
		mutex.lock
		var r = super
		mutex.unlock
		return r
	end

	redef fun shift
	do
		mutex.lock
		var value = real_collection.shift
		mutex.unlock
		return value
	end

	# If empty, blocks until an item is inserted with `push` or `unshift`
	redef fun shift do
		mutex.lock
		while real_collection.is_empty do self.cond.wait(mutex)
		var r = real_collection.shift
		mutex.unlock
		return r
	end

	# If empty, blocks until an item is inserted with `push` or `unshift`
	redef fun shift do
		mutex.lock
		if real_collection.is_empty then
			actor.working = false
			sys.active_actors.decrement
			while real_collection.is_empty do self.cond.wait(mutex)
		end
		var r = real_collection.shift
		mutex.unlock
		return r
	end