frameset.go
func (s *FrameSet) Frame(index int) (int, error) {
	return s.rangePtr.Value(index)
}
func (s *FrameSet) Frame(index int) (int, error) {
	return s.rangePtr.Value(index)
}
// Frame returns the frame number value for a given index into
// the frame set.
// If the index is outside the bounds of the frame set range,
// then an error is returned.