code: public Version append(int number, String label) {
          validateNumber(number);
          return appendNumber(number, label);
      }
docs: Adds a new version number.
  
  @param number version number (not negative).
  @param label version number label (optional).
  @return version descriptor.
  @throws IllegalArgumentException if version number is illegal.
  
  @since v1.1.0