  In our previous series of studies to investigate the role of evidential
reasoning in the RUBRIC system for full-text document retrieval (Tong et al.,
1985; Tong and Shapiro, 1985; Tong and Appelbaum, 1987), we identified the
important role that problem structure plays in the overall performance of the
system. In this paper, we focus on these structural elements (which we now call
"semantic structure") and show how explicit consideration of their properties
reduces what previously were seen as difficult evidential reasoning problems to
more tractable questions.
