Cathy Sarisky, I was thinking about the category cross-over problem you mention, also, because any attempt to restrict categorization organization tends to cause points of stress in a system, which shows itself in other ways. The 1 parent rule has important performance implications in object-modeling, so it would be best to keep it if we can.
Re-organizing categories seems to work within the proposal for anything I can think of right now. In your example, having Biology and Chemistry as childs (children) of Science (and furthermore Science a child of Everything), allows Biochemistry to be a member of Biology and Chemistry --without breaking the requirement of membership from different tree branches. Using the categories on your website, one could have Engineering (application of science) as a child of Science or Everything. A Science Fair Biochemistry Project category might share membership with Engineering, Biology and Chemistry --again, not breaking the requirements.
Neophytos Demetriou, would there/could there be a mechanism to check for overlapping category membership (for people like me who tend to get into categorization mazes), or does addressing this become part of OACS internal configurations and administrating level, and therefore a much lower development priority?
