// MoveBefore moves element e to its new position before mark.
// If e or mark is not an element of l, or e == mark, the list is not modified.
func (l *List) MoveBefore(e, mark *Element) {
    if e.list != l || mark.list != l  || e == mark || e == mark.prev{
        return
    }
    l.insert(l.remove(e), mark.prev)
}

// MoveAfter moves element e to its new position after mark.
// If e or mark is not an element of l, or e == mark, the list is not modified.
func (l *List) MoveAfter(e, mark *Element) {
    if e.list != l || mark.list != l || e == mark || e == mark.next {
        return
    }
    l.insert(l.remove(e), mark)
}