1. A method for presenting materials corresponding to a navigation state, the method comprising: receiving a user selection of an expression of attribute-value pairs; producing a plurality of refinement options and a plurality of ancestors by processing, in each server of a plurality of servers, the expression of attribute-value pairs to produce at least one refinement option and at least one ancestor; combining the plurality of refinement options and plurality of ancestors to form combined refinement options, the combined refinement options including at least one refinement navigation state; determining the navigation state associated with the expression of attribute-value pairs; providing materials associated with the navigation state; and providing the combined refinement options, wherein the combining comprises: taking a union of the plurality of refinement options, determining a set of ancestors for each refinement option of the plurality of refinement options, from the plurality of ancestors produced in the plurality of servers, to form sets of ancestors, computing an intersection of all of the sets of ancestors, and computing the combined refinement options based on terms in the intersection of all sets of ancestors, including identifying at least two related terms among the plurality of refinement options, and computing, for the at least two related terms, a least common ancestor of the related terms, and wherein a first server of the plurality of servers acts as a master server and some of the plurality of servers act as slave servers, the method further comprising the master server distributing a request for a navigation state to a plurality of slave servers, the slave servers computing navigation states for requests and returning results to the master server, and the master server combining the results from the slave servers to obtain a navigation state corresponding to the request, wherein the combining of the results is based on the combining of the plurality of refinement options and plurality of ancestors.
1. A method for presenting materials corresponding to a navigation state, the method comprising: receiving a user selection of an expression of attribute-value pairs; producing a plurality of refinement options and a plurality of ancestors by processing, in each server of a plurality of servers, the expression of attribute-value pairs to produce at least one refinement option and at least one ancestor; combining the plurality of refinement options and plurality of ancestors to form combined refinement options, the combined refinement options including at least one refinement navigation state; determining the navigation state associated with the expression of attribute-value pairs; providing materials associated with the navigation state; and providing the combined refinement options, wherein the combining comprises: taking a union of the plurality of refinement options, determining a set of ancestors for each refinement option of the plurality of refinement options, from the plurality of ancestors produced in the plurality of servers, to form sets of ancestors, computing an intersection of all of the sets of ancestors, and computing the combined refinement options based on terms in the intersection of all sets of ancestors, including identifying at least two related terms among the plurality of refinement options, and computing, for the at least two related terms, a least common ancestor of the related terms, and wherein a first server of the plurality of servers acts as a master server and some of the plurality of servers act as slave servers, the method further comprising the master server distributing a request for a navigation state to a plurality of slave servers, the slave servers computing navigation states for requests and returning results to the master server, and the master server combining the results from the slave servers to obtain a navigation state corresponding to the request, wherein the combining of the results is based on the combining of the plurality of refinement options and plurality of ancestors. 37. The method of claim 1 , further comprising storing at least a portion of the plurality of ancestors produced in the plurality of servers in a cache.