queue:
	{ :start 0 :end 0 }

enqueue q item:
	set-to q q!end item
	set-to q :end ++ q!end

dequeue q:
	if empty q:
		Raise :value-error "popping from empty queue"
	q! q!start
	delete-from q q!start
	set-to q :start ++ q!start

empty q:
	= q!start q!end