An augmentation of semantic networks is presented in which the various nodes and arcs are partitioned into "net spaces." These net spaces delimit the scopes of quantified variables, distinguish hypothetical and imaginary situations from reality, encode alternative worlds considered in planning, and focus attention at particular levels of detail.
R. Brachman, and D. McGuinness. SIGIR '88: Proceedings of the 11th annual international ACM SIGIR conference on Research and development in information retrieval, page 161--174. New York, NY, USA, ACM, (1988)