code: func (fs *MockFilesystem) Add(name string) *MockFile {
  	f := &MockFile{
  		Seq: -1,
  		fd:  uintptr(time.Now().UnixNano() & 0xffff),
  		fs:  fs,
  	}
  	fs.Files[name] = f
  	return f
  }
docs: // Add adds a new file to fs.Files given a name, and returns the newly created file