code: func (f *FindFilter) RunFilter(arg Arg) error {
  	return filepath.Walk(f.dir, func(n string, s os.FileInfo, e error) error {
  		if e != nil {
  			return e
  		}
  		if s.Mode().IsDir() && f.skipdirif(n) {
  			return filepath.SkipDir
  		}
  		if f.ifmode(s.Mode()) {
  			arg.Out <- n
  		}
  		return nil
  	})
  }
docs: // RunFilter yields contents of a filesystem tree. It implements the
  // Filter interface.