sequence.go
func (s *FileSequence) Len() int {
	if s.frameSet == nil {
		return 1
	}
	return s.frameSet.Len()
}
func (s *FileSequence) Len() int {
	if s.frameSet == nil {
		return 1
	}
	return s.frameSet.Len()
}
// Len returns the number of frames in the FrameSet.
// If a frame range was not parsed, this will always return 1