protected void append(Link link) {
link.prev = this;
link.next = this.next;
this.next = link;
if (link.next != null) {
link.next.prev = link;
}
}
protected void insert(Link link) {
link.next = this;
link.prev = this.prev;
this.prev = link;
if (link.prev != null) {
link.prev.next = link;
}
}
