frameset.go
func (s *FrameSet) Index(frame int) int {
	return s.rangePtr.Index(frame)
}
func (s *FrameSet) Index(frame int) int {
	return s.rangePtr.Index(frame)
}
// Index returns the index position of the frame value
// within the frame set.
// If the given frame does not exist, then return -1