# Append `l` to `self` but clear `l`.
	#
	# O(1)
	fun link(l: List[E])
	do
		var tail = _tail
		if tail == null then
			_head = l._head
		else if l._head != null then
			tail.next = l._head
			tail.next.as(not null).prev = tail
		end
		_tail = l._tail
		length += l.length
		l.clear
	end