code: func (fs *MockFilesystem) OpenFile(name string, flag int, perm os.FileMode) (file File, err error) {
  	f, ok := fs.Files[name]
  	if ok {
  		f.Opened = true
  		f.Closed = false
  		return f, nil
  	}
  	return (*MockFile)(nil), &os.PathError{Err: errors.New(name + ": No such file.")}
  }
docs: // OpenFile opens file name from fs.Files, if the file does not exist it returns an os.PathError