func (l *List) PushFront(e Element) {
	ElementMapper{}.linkerFor(e).SetNext(l.head)
	ElementMapper{}.linkerFor(e).SetPrev(nil)
	if l.head != nil {
		ElementMapper{}.linkerFor(l.head).SetPrev(e)
	} else {
		l.tail = e
	}
	l.head = e
}
PushFront inserts the element e at the front of list l.
package ilist
type Linker interface {
	Next() Element
	Prev() Element
	SetNext(Element)
	SetPrev(Element)
}
type Element interface {
	Linker
}
type ElementMapper struct{}
func (ElementMapper) linkerFor(elem Element) Linker { return elem }
type List struct {
	head Element
	tail Element
}
func (l *List) Reset() {
	l.head = nil
	l.tail = nil
}
func (l *List) Empty() bool {
	return l.head == nil
}
func (l *List) Front() Element {
	return l.head
}
func (l *List) Back() Element {
	return l.tail
}