code: func (a luhnAlphabet) luhnValidate(s string) bool {
  	t := s[:len(s)-1]
  	c, err := a.generate(t)
  	if err != nil {
  		return false
  	}
  	return rune(s[len(s)-1]) == c
  }
docs: // luhnValidate returns true if the last character of the string s is correct, for
  // a string s composed of characters in the alphabet a.