code: public List<String> getPreReleaseLabels() {
      return preReleases == null ? null : Collections.unmodifiableList(Arrays.asList(preReleases));
    }
docs: The pre-release labels if relevant, i.e. label1 and label2 in X.Y.Z-label1-lable2.
  
  @return the pre-release labels. The return list will be {@code null} if the version number
  doesn't have any.