func (s *Iterator) Prev() {
	y.AssertTrue(s.Valid())
	s.n, _ = s.list.findNear(s.Key(), true, false) // find <. No equality allowed.
}
func (s *Iterator) Prev() {
	y.AssertTrue(s.Valid())
	s.n, _ = s.list.findNear(s.Key(), true, false) 
}
func ( s * Iterator ) Prev ( ) { y . AssertTrue ( s . Valid ( ) ) s . n , _ = s . list . findNear ( s . Key ( ) , true , false ) // find <. No equality allowed. }
// Prev advances to the previous position.
Prev advances to the previous position .