1. At least one computer-readable storage medium having computer-executable instructions stored thereon which, when executed by a computer system, cause the computer system to perform a method comprising: receiving at least one natural language input query; defining at least one term occurrence-level constraint applicable to the input query; identifying at least one fact requested in the input query; instantiating at least one iterator to traverse at least one fact index to identify at least one candidate fact that is responsive to the input query; traversing the iterator through at least a portion of the fact index; receiving at least one candidate fact from the iterator, wherein the candidate fact includes at least one term occurrence; applying the term occurrence-level constraint to the term occurrence; and selecting the candidate fact for inclusion in search results for the input query, based at least in part on applying the term occurrence-level constraint to the term occurrence.