In order to maintain the integrity of merged topic maps, it is necessary to establish a common structure for all assertions. In this RM4TM, the decisions as to which aspects of the structure of assertions should be "reified" as nodes, and which aspects should remain "unreified" as arcs, were made by distinguishing between the aspects of assertions that are substantive with respect to the relationships that they assert (and that could conceivably, therefore, need to become role players in other assertions about those relationships), as opposed to the aspects of assertions that nobody would want to make other assertions about unless they were discussing the design of assertions in general. In the structure of assertions set forth in this RM4TM, the former aspects are represented by a-nodes and c-nodes, while the latter aspects are represented as the four types of arcs (the "eight forms of connectedness").

Need to revisit definition of arc to say that it is an unreified
attribution of roles in an assertion. Thus, both arcs and edges in an
assertion are unreified, versus reified aspects.