code: func (x *Exif) Walk(w Walker) error {
  	for name, tag := range x.main {
  		if err := w.Walk(name, tag); err != nil {
  			return err
  		}
  	}
  	return nil
  }
docs: // Walk calls the Walk method of w with the name and tag for every non-nil
  // EXIF field.  If w aborts the walk with an error, that error is returned.