12. A system, comprising: a query logic to receive a query comprising one or more query terms; an index logic, embodied at least in part on a non-transitory computer-readable medium, to store a set of index entries and to provide access to data stored in the set of index entries, an index entry comprising a first identifier that identifies a first document in which a term appears, and a second identifier that identifies a second document that is related to the first document; an inclusion logic, embodied at least in part on the non-transitory computer-readable medium, to produce a virtual document that includes a base level document and a first level document, the base level document including at least one of the one or more query terms, the first level document being related to the base level document by the second identifier; and a relevance logic, embodied at least in part on the non-transitory computer-readable medium, to determine the relevance of the virtual document to the query.
19. The system of claim 12 , comprising: a presentation logic to present data concerning a base document relevant to the query and concerning a first level document related to the base document, where the first level document is related to the base level document by the second identifier, the first level document being related to the base level document as one or more of, an attached document, an attaching document, a referenced document, and a referencing document, where the virtual document selectively includes a second level document related to a first level document as one or more of, an attaching document, an attached document, a referencing document, and a referenced document; where the relevance logic determines the relevance of the virtual document to the query using a term frequency value, an inverse document frequency value, and a nearness value, where the nearness value depends, at least in part, on whether two terms appearing in the virtual document appear in an individual document that comprises a portion of the virtual document.