# Search `key` in `from` and its children nodes.
	protected fun search_down(from: N, key: nullable Object): nullable N do
		if not key isa Comparable then return null
		var cmp = key <=> from.key
		if cmp == 0 then return from
		if from.left != null and cmp < 0 then
			return search_down(from.left.as(not null), key)
		else if from.right != null then
			return search_down(from.right.as(not null), key)
		end
		return null
	end