As an example, reconsider the hypermedia scheme specified in Fig. 1. This scheme allows for the maintenance of different
versions of info nodes. Now consider Fig. 17. This figure displays a subinstance of a hypermedia object base instance
different from the one displayed in Figs. 2 and 3. As can be seen, the info nodes pointed at by the version nodes share
info nodes to which they are linked. In fact, in some cases,info nodes share the same set of other info nodes, as do, for
instance, the first and second info nodes from the left. In order to “abstract” over info nodes that share the same set of nodes, consider Fig. 18. This figure contains two node additions and an abstraction operation. The two node additions are used to tag the info nodes over which the abstraction will take place. An abstraction operation consists of three distinguishable parts.
The first part (in solid lines) is the source pattern.
The second part (in dashed lines) specifies the type of set equality (i.e., info nodes are grouped together if they are linked to the same set of info nodes). The third part (in bold lines) specifies the type of nodes and edges to be added as the result of the abstraction operation. The semantics of this operation is simply that for each group of info nodes being linked to the same set of info nodes, a new node with label same-info is introduced and linked to all the members of the group by edges with label contains. The result of this operation is shown in Fig. 19.