docs: public static TagModel getSingleParent(TagModel tag)
      {
          final Iterator<TagModel> parents = tag.getDesignatedByTags().iterator();
          if (!parents.hasNext())
              throw new WindupException("Tag is not designated by any tags: " + tag);
  
          final TagModel maybeOnlyParent = parents.next();
  
          if (parents.hasNext()) {
              StringBuilder sb = new StringBuilder();
              tag.getDesignatedByTags().iterator().forEachRemaining(x -> sb.append(x).append(", "));
              throw new WindupException(String.format("Tag %s is designated by multiple tags: %s", tag, sb.toString()));
          }
  
          return maybeOnlyParent;
      }