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Structure of a DTD
A DTD is based on strict rules defined in the SGML standard. Arbortext IsoDraw uses a subset of the possibilities that are available in SGML. The main rules are described below.
A DTD is saved as a text file with extension .dtd. Arbortext IsoDraw uses the file name without the extension for the name of the DTD.
The names _DEFAULT_DTD_, grexchange and webcgm are reserved. The existing DTDs should not be changed, otherwise CGM export may no longer agree with the relevant standards.
A DTD can also contain annotations which are ignored for interpretation purposes. An annotation begins with the characters <!-- and ends with characters -->. It can encompass several lines.
Example:
<!-- This is an annotation -->
The following definitions may appear in a DTD
ELEMENT
Corresponds to the object type
ATTLIST
The list of attributes
ENTITY
A type of macro which defines expressions used in other definitions