# Remove `line` from this block.
	fun remove_line(line: MDLine) do
		if line.prev == null then
			first_line = line.next
		else
			line.prev.as(not null).next = line.next
		end
		if line.next == null then
			last_line = line.prev
		else
			line.next.as(not null).prev = line.prev
		end
		line.prev = null
		line.next = null
	end