class MeetingPlace
	var meetings_left: Int
	var firstcolor: nullable Int
	var firstid: Int = 0
	var current: Future[Pair] is noinit

	private var mutex = new Mutex

	fun meet(id, c: Int): nullable Pair do
		var new_pair = new Future[Pair]
		mutex.lock
		if meetings_left == 0 then
			mutex.unlock
			return null
		else
			if firstcolor == null then
				firstcolor = c
				firstid = id
				current = new Future[Pair]
			else
				var color = complements[c][firstcolor.as(not null)]
				current.set_value(new Pair(id == firstid, color))
				firstcolor = null
				meetings_left -= 1
			end
			new_pair = current
		end
		mutex.unlock
		return new_pair.join
	end
end