docs: public Tag getOrCreateTag(String tagName, boolean isRef)
      {
          if (null == tagName)
              throw new IllegalArgumentException("Looking for a null tag name.");
          tagName = Tag.normalizeName(tagName);
  
          synchronized (this.definedTags)
          {
              if (definedTags.containsKey(tagName))
                  return definedTags.get(tagName);
              else
              {
                  final Tag tag = new Tag(tagName);
                  definedTags.put(tagName, tag);
                  return tag;
              }
          }
      }