" Each category can only have one parent. "
Is this essential? I have some categories that would really be most appropriately assigned to two parents. For example, www.labarchive.net (still running 3.2.5) uses categories to classify uploaded experiments. Sometimes having two parents for a category would make more sense. For example, a biochemistry category might most appropriately be listed as a child of BOTH chemistry and biology. Similarly for other interdisciplinary children, joint ventures, etc.
