code: func isLabel(s string) bool {
  	if len(s) == 0 || !isAlpha(rune(s[0])) {
  		return false
  	}
  	for _, c := range s[1:] {
  		if !isAlphaNumeric(c) {
  			return false
  		}
  	}
  	return true
  }
docs: // isLabel reports whether the string can be used as label.