# Return the value associated to the keys `k1` and `k2`.
	# Return `null` if no such a value.
	fun [](k1: K1, k2: K2): nullable V
	do
		var level1 = self.level1
		var level2 = level1.get_or_null(k1)
		if level2 == null then return null
		return level2.get_or_null(k2)
	end