# Remove the item at `k1`, `k2`, `k3` and `k4`
	fun remove_at(k1: K1, k2: K2, k3: K3, k4: K4)
	do
		var level1 = self.level1
		var level2 = level1.get_or_null(k1)
		if level2 == null then return
		level2.remove_at(k2, k3, k4)
	end