code: func MakeReadOnly(path string, filter func(string) bool) error {
  	return recursiveChmod(path, filter, func(mode os.FileMode) os.FileMode {
  		return mode & (^os.FileMode(0222))
  	})
  }
docs: // MakeReadOnly recursively iterates through all of the files and directories
  // starting at path and marks them read-only.