# Set `v` the value associated to the keys `k1`, `k2`, `k3` and `k4`.
	fun []=(k1: K1, k2: K2, k3: K3, k4: K4, v: V)
	do
		var level1 = self.level1
		var level2 = level1.get_or_null(k1)
		if level2 == null then
			level2 = new HashMap3[K2, K3, K4, V]
			level1[k1] = level2
		end
		level2[k2, k3, k4] = v
	end