func (l *segmentList) Front() *segment {
	return l.head
}
Front returns the first element of list l or nil.
package tcp
type segmentElementMapper struct{}
func (segmentElementMapper) linkerFor(elem *segment) *segment { return elem }
type segmentList struct {
	head *segment
	tail *segment
}
func (l *segmentList) Reset() {
	l.head = nil
	l.tail = nil
}
func (l *segmentList) Empty() bool {
	return l.head == nil
}