built-in entity
external-text-entity #document
Provides the framework for explicitly providing an entire "SGML document entity" to the SGML parser and is used to control where input comes from in a down-translation.
The external-text-entity #document rule differs from the other external-text-entity rules in the following ways:
#document cannot be mixed with entity names or with #dtd, #charset, #capacity, or #syntax in the header of the rule (for example, external-text-entity #document | #dtd is not allowed).
external-text-entity #document rules in the program, one, and only one, must be selected to provide the SGML document entity. (An error will be issued if none or more than one is selected.)
external-text-entity #document rule must not contain any find-start or find-end rules (there is no reason for them to be performed).
external-text-entity #document rule only applies to the main SGML document, and not to documents whose parsing was initiated by a do sgml-parse. Thus, the #current-output inherited by an external-text-entity #document rule is always the #sgml stream created by the context-translate, down-translate, or up-translate translation type.
external-text-entity #document because this type of text entity does not have a name.