func (l *List) PushBackList(l2 *List) {
	l2.dummy.Next.Prev = l.dummy.Prev
	l.dummy.Prev.Next = l2.dummy.Next
	l2.dummy.Prev.Next = l.dummy
	l.dummy.Prev = l2.dummy.Prev
	l.N += l2.N
}
//PushBackList inserts an existing List l2 at the back of list l.
//l2 must not be used after this.

PushBackList inserts an existing List l2 at the back of list l.
l2 must not be used after this.
// PushBackList inserts an existing List l2 at the back of list l . // l2 must not be used after this .
func (l *List) PushBackList(l2 *List) {
	l2.dummy.Next.Prev = l.dummy.Prev
	l.dummy.Prev.Next = l2.dummy.Next
	l2.dummy.Prev.Next = l.dummy
	l.dummy.Prev = l2.dummy.Prev
	l.N += l2.N
}
func ( l * List ) PushBackList ( l2 * List ) { l2 . dummy . Next . Prev = l . dummy . Prev l . dummy . Prev . Next = l2 . dummy . Next l2 . dummy . Prev . Next = l . dummy l . dummy . Prev = l2 . dummy . Prev l . N += l2 . N }
PushBackList inserts an existing List l2 at the back of list l.
l2 must not be used after this.
PushBackList inserts an existing List l2 at the back of list l . l2 must not be used after this .