code: func checkPairs(pairs ...string) (int, error) {
  	length := len(pairs)
  	if length%2 != 0 {
  		return length, fmt.Errorf(
  			"mux: number of parameters must be multiple of 2, got %v", pairs)
  	}
  	return length, nil
  }
docs: // checkPairs returns the count of strings passed in, and an error if
  // the count is not an even number.