The intersection, S1 ∩ S2 ∩ · · · ∩ Sn, of the sets gives the document
identifiers of the desired set of documents. The or operation gives the set of all
documents that contain at least one of the keywords K1, K2, . . . , Kn. We implement
the or operation by computing the union, S1 ∪ S2 ∪ · · · ∪ Sn, of the sets.
The not operation finds documents that do not contain a specified keyword Ki .
Given a set of document identifiers S, we can eliminate documents that contain
the specified keyword Ki by taking the difference S − Si, where Si is the set of
identifiers of documents that contain the keyword Ki