private fun replace_child(old_child: ANode, new_child: nullable ANode): Bool
	do
		var parent = self.parent
		for i in [0..length[ do
			if self[i] == old_child then
				if new_child != null then
					assert new_child isa E
					self[i] = new_child
					new_child.parent = parent
				else
					self.remove_at(i)
				end
				return true
			end
		end
		return false
	end