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While processing an SGML document the document may refer to external
data. This occurs in three places: external parameter entities, normal
external entities and the
DOCTYPE declaration. The current
version of this tool deals rather primitively with external data.
External entities can only be loaded from a file and the mapping between
the entity names and the file is done using a catalog file in a
format compatible with that used by James Clark's SP Parser, based on
the SGML Open (now OASIS) specification.
Catalog files can be specified using two primitives: the predicate
or the environment variable
SGML_CATALOG_FILES (compatible with the SP package).
endand defines whether the catalog is considered first or last. This predicate has no effect if File is already part of the catalog.
If no files are registered using this predicate, the first query on
the catalog examines
SGML_CATALOG_FILES and fills the
catalog with all files in this path.
Two types of lines are used by this package.
The specified file path is taken relative to the location
of the catolog file. For the
first makes an attempt to resolve the
identifier. If this fails it tries to resolve the doctype
using the provided catalog files.
library(sgml) breaks the rules for
XML, where system identifiers must be Universal Resource Indicators, not
local file names. Simple uses of relative URIs will work correctly under
UNIX and Windows.
In the future we will design a call-back mechanism for locating and processing external entities, so Prolog-based file-location and Prolog resources can be used to store external entities.