 network levels. That does not matter at all, however, since the complexity of a system/model is not measured in terms of the number of elements, but by their relationships. By introducing hosts and term relations we have been able to dissolve an otherwise impenetrable mass of allo relations (compare with Fig. 4.3) into much fewer relations of three different types. In a sense, this type of model summarizes almost everything there is to say about structures in network systems. Besides being an excellent tool for require- ment tracing and network configuration, it gives strong support for the evolution of network systems and networks. For example, if we would like to replace the inter- 4 Modeling Vertical and Horizontal Partitions nal structure of n1.2.1 with a single processor node, the only model editing required is to remove the sub relation in n1.2.1. The modeling tool would be able to draw the following conclusions as a result of such a request. The network nw2.2 will disappear from the model. As a result, the term relations for n1.3.1 and n1.4.1 will disappear All hosts relations from nw2.1 will disappear as well. Since the model includes no hosts relations from nw2.1 to nodes of other net- works, nw2.1 will also disappear. As a consequence the allo relation from nw2.1 to the layer LD will disappear. The allo relation from nw2.2 to the common agent will disappear. This leaves us with a model that comprises network level 1 only in this case. Note, however, that in another case there may exist many sub relations that repre- sent the relation between the two adjacent network levels, in which case all such relations must be removed in order to return to the network level 1 model. This calls for defining a substructure relation identifier (SRi) that, for each relation, identifies the adjacent network levels. In the model of Fig. 4.16 we have annotated the sub relation between n1.2.1 an n2.2.1 as "1/2" in order to tell that the relation goes between network level 1 and 2. Using SRi annotations in complex models im