The notion of ``proximity'', which leads us to the idea of arrangement or grouping, is crucial for the classification problem, in general, and for document organization and retrieval in particular. As documents become more and more the center of computer activity, it will be of a dramatic importance its identification, store, track, retrieve and presentation [13].
The COMPARATOR subsystem of SOUR[14] is encharged of maintaining an ordered structure of AOs which are grouped hierarchically according to a ``proximity'' order defined on the system's standard of conceptualization. It performs the crucial task of comparing Abstract Objects (AOs), while providing a meaningful decision procedure for AO-equivalence. The relevance of COMPARATOR cannot be underestimated - it amounts to the definition of AO-semantics itself, based upon the belief that document semantics can be effectively captured by the adopted attributive model.
The standard attribute-based comparison, at coarse level, is present as a preliminary, less discriminant decision procedure. But with such a procedure the expressive power of the system does not go beyond the conventional, object-oriented information model.
The desired increase in expressive power is achieved at fine level, where object comparison is ``fuzzy'' and is decided according to a metrics or algebra of proximity which computes intersections of the proximity closures of facet values within their hierarchical conceptual graphs. For the technical aspects of this sophisticated tool, see [14] and [10].