The Visual Specification of Context

We introduce a new visual technique for the specification of context in hierarchically structured documents such as those defined by XHTML and XML. The technique is based on a formal graph model called T-graphs and on what we call T-configurations in (abstract) document trees. A T-configuration is a restricted substructure of a tree. The technique is implemented in the current version of Designer. Although we apply this technique to the specification of context-dependent stylesheets for XHTML and XML documents, the technique has much wider applicability. For example, it can be applied to query specification for structured documents and to computer program transformations. Previously, we introduced a context-specification method that used regular expressions to specify sets of paths in a tree that we called caterpillar expressions. The work on T-graphs is an attempt to provide a 90 percent solution to context-specification problems that solves, in practice, almost all context-specification problems. T-graphs are much easier to visualize and to construct than are caterpillar expressions; they are both a restriction of and a generalization of caterpillar expressions. We compare T-graphs with the context specification techniques found in other stylesheet systems and we also provide examples of context that we can and cannot specify with T-graphs. Although T-graphs are restrictive, they lend themselves to visual construction and modification, our main requirement when we designed this context-specification method. We also investigate the time and space complexity of T-graph matching, a necessity for efficient implementation.