Suggest how a document containing a word (such as “leopard”) can be indexed such that it is efficiently retrieved by queries using a more general concept (such as “carnivore” or “mammal”). You can assume that the concept hierarchy is not very deep, so each concept has only a few generalizations (a concept can, however, have a large number of specializations). You can also assume that you are provided with a function that returns the concept for each word in a document. Also suggest how a query using a specialized concept can retrieve documents using a more general concept.