built-in data type
The abstract type dtd represents parsed SGML and XML DTDs. It has two subtypes, sgml-dtd and
xml-dtd, corresponding to the two types of DTDs created by the SGML and XML parser.
Elements and events declared by the DTD can be extracted using the operators declared-elements,
declared-general-entities, and declared-parameter-entities.
Built-in shelves sgml-dtds and xml-dtds are of type sgml-dtd and xml-dtd,
respectively. The built-in dtd shelf #current-dtd also contains a value of one of these two
types during a validating SGML or XML parse.