code: func (f *file) String() string {
  	var buf bytes.Buffer
  	buf.WriteString(f.name)
  	buf.WriteRune(fileStartMark)
  	buf.WriteString(f.contents)
  	buf.WriteRune(fileEndMark)
  
  	return buf.String()
  }
docs: // String returns a string formated as: name<contents>.