# Insert `sibling` before `self`.
	fun insert_before(sibling: MdNode) do
		sibling.unlink
		sibling.prev = prev
		if sibling.prev != null then
			sibling.prev.as(not null).next = sibling
		end
		sibling.next = self
		prev = sibling
		sibling.parent = parent
		if sibling.prev == null then
			sibling.parent.as(not null).first_child = sibling
		end
	end