func (m *Mapping) OrigSource() (string, int64) {
	if m.origF == nil {
		return "", 0
	}
	return m.origF.Name(), m.origOff
}
func (m *Mapping) OrigSource() (string, int64) {
	if m.origF == nil {
		return "", 0
	}
	return m.origF.Name(), m.origOff
}
// For CopyOnWrite mappings, OrigSource returns the file/offset of the
// original copy of the data, or "", 0 if none.