1. A computer-implemented method, comprising: receiving, from a user, a search request specifying a topic, wherein the topic is included in an ontology, wherein the ontology specifies that the topic is divided into a plurality of subtopics, and wherein the search request does not specify any of the subtopics; by operation of one or more computer processors, identifying one or more documents residing on a network based on the topic and an ontology index, wherein the ontology index associates the topic with: (i) positions within the documents, at which the topic appears and (ii) positions within the documents, at which any of the subtopics appears; and responsive to the search request from the user, presenting one or more search results that describe the one or more documents to the user, wherein at least one of the one or more documents contains at least one of the subtopics and does not contain the topic.