code: func parentDirs(rel string, cb func(p string) bool) {
  	for i, r := range rel {
  		if r == filepath.Separator && !cb(rel[:i]) {
  			return
  		}
  	}
  }
docs: // parentDirs takes a native path "a/b/c/d" and calls 'cb' with "a", "a/b"
  // and "a/b/c".
  //
  // Purely lexicographical operation.
  //
  // Stops if the callback returns false. Note that it doesn't visit 'rel' itself.