code: func IsAlpha(s string) bool {
  	for _, v := range s {
  		if ('Z' < v || v < 'A') && ('z' < v || v < 'a') {
  			return false
  		}
  	}
  	return true
  }
docs: // IsAlpha check if the string contains only letters (a-zA-Z). Empty string is valid.