code: func enforceSingleSub(regexp *syntax.Regexp) error {
  	if len(regexp.Sub) != 1 {
  		return generatorError(nil,
  			"%s expected 1 sub-expression, but got %d: %s", opToString(regexp.Op), len(regexp.Sub), regexp)
  	}
  	return nil
  }
docs: // Return an error if r has 0 or more than 1 sub-expression.