Taking SunirShah's definition of an idea from CollectiveIdea, an idea can be subclassed.

Staying true to that definition the subclass then can only be another idea. The new idea has been derived from the original idea. The original idea is no longer just another idea, but is now an IdeaFragment. for example: The internal combustion engine is a relatively recent IdeaFragment (historically speaking)that combined with the relatively old IdeaFragment of the wheel produced the idea of the automobile. --DanHerrington