code: func NewFileFragment(f *os.File, startLoc, endLoc int64) (*FileFragment, error) {
  	thisF, err := os.Open(f.Name())
  	if err != nil {
  		return nil, errors.Wrap(err, "opening file fragment")
  	}
  	_, err = thisF.Seek(startLoc, io.SeekStart)
  	if err != nil {
  		return nil, errors.Wrap(err, "seeking to start location in new file handle")
  	}
  	return &FileFragment{
  		file:     thisF,
  		startLoc: startLoc,
  		endLoc:   endLoc,
  	}, nil
  }
docs: // NewFileFragment returns a FileFragment which will read only from startLoc to
  // endLoc in a file.