func (l *PacketList) Back() *PacketElement {
	if l.len == 0 {
		return nil
	}
	return l.root.prev
}
Back returns the last element of list l or nil if the list is empty.
package ackhandler
type PacketElement struct {
	next, prev *PacketElement
	list *PacketList
	Value Packet
}
func (e *PacketElement) Next() *PacketElement {
	if p := e.next; e.list != nil && p != &e.list.root {
		return p
	}
	return nil
}
func (e *PacketElement) Prev() *PacketElement {
	if p := e.prev; e.list != nil && p != &e.list.root {
		return p
	}
	return nil
}
type PacketList struct {
	root PacketElement  
	len  int            
}
func (l *PacketList) Init() *PacketList {
	l.root.next = &l.root
	l.root.prev = &l.root
	l.len = 0
	return l
}
func NewPacketList() *PacketList { return new(PacketList).Init() }
func (l *PacketList) Len() int { return l.len }
func (l *PacketList) Front() *PacketElement {
	if l.len == 0 {
		return nil
	}
	return l.root.next
}