Metamarkup. Finally, metamarkup provides authors and support personnel with a facility for controlling the interpretation of markup and for extending the vocabulary of descriptive markup languages. Procedural and descriptive systems provide ways to define markup delimiter characters. In addition, procedural systems include such instructions as define macro, which are typically used to create descriptive markup representing a series of processing instructions. The procedural markup in Figure 1, for example, would typically be included in macros with names such as quo and quoend. Applications that process GML, such as Waterloo SCRIPT, also provide markup to define tags, to specify valid and default attributes, and to indicate what instructions should be executed when the tag is encountered. Finally, in SGML, metamarkup appears in the form of “markup declarations,” of which there are 13 kinds.

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