Sgml and Exceptions 1 Pekka Kilpell Ainen 2

The Standard Generalized Markup Language (SGML) allows users to deene document type deenitions (DTDs), which are essentially extended context-free grammars in a notation that is similar to extended Backus{Naur form. The right-hand side of a production is called a content model and its semantics can be modiied by exceptions. We give precise deenitions of the semantics of exceptions and prove that they do not increase the expressive power of SGML. For each DTD with exceptions we can construct a structurally equivalent extended context-free grammar. On the other hand, exceptions are a powerful shorthand notation|eliminating them may cause exponential growth in the size of a DTD.