SGML can refer to an external file (really entity) with an external identifier, this is a public identifier or a system identifier, or both.
A typical public identifier looks like
PUBLIC "ISO 8879:1986//ENTITIES Added Latin 1//EN"
where “ISO 8879:1986” is the owner, “ENTITIES” is the text class and “Added Latin 1” is the text description (and “EN” is language).
A system identifier looks like
SYSTEM "htmlplus.dtd"
where “htmlplus.dtd” is a system-specific identifier.
To map external identifiers to file names, PSGML first searches entity
catalog files and then search the list of file name templates in the
variable sgml-public-map.
The catalog format is according to SGML/Opens resolution on entity management. The catalog consists of a series of entries and comments. A comment is delimited by ‘--’ like in a markup declaration. The entry types recognized are described in the following table.
The file will be used for the entity text of an entity with the public identifier pubid.
The file will be used for the entity text of an entity with the name name. If the name starts with a ‘%’ the rest of the name will be matched against parameter entities.
The file will be used for the entity text of an entity used as external subset of a document declaration with name as document type name.
Used to specify a default SGML declaration. Recognized but not used by
PSGML other than to pass to an external validation command
(sgml-validate-command).
When PSGML is looking for the file containing an external entity, the following things will be tried in order:
sgml-system-identifiers-are-preferred
is non-nil and there is no elements containing ‘%s’ in
sgml-public-map. If the system identifier is a relative file name
it will be relative to the directory containing the defining entity.
sgml-local-catalogs and
sgml-catalog-files in order. For each catalog look first for
entries matching the public identifier, if any. Then look for other
matching entries in the order they appear in the catalog.
Currently an entry will be ignored if it is matching but its file is non-existent or unreadable. (This is under reconsideration, perhaps it should signal error instead).
sgml-system-identifiers-are-preferred is nil
and there is no elements containing ‘%s’ in sgml-public-map.
sgml-public-map. Using the catalogs are
preferred. The sgml-public-map may disappear in a future version
of PSGML (not soon though).
The sgml-public-map variable can contain a list of file name
templates where ‘%P’ will be substituted with the whole public
identifier, owner is substituted for ‘%O’, public text class for
‘%C’, and public text description for ‘%D’. The text class
will be converted to lower case and the owner and description will be
transliterated according to the variable
sgml-public-transliterations. The templates in the list is tried
in order until an existing file is found. The sgml-public-map is
modeled after sgmls environment variable SGML_PATH and
psgml understand the following substitution characters: %%, %N, %P, %S,
%Y, %C, %L, %O, %T, and %V. The default value of
sgml-public-map is taken from the environment variable
SGML_PATH.
Given the public identifier above and the file name template ‘/usr/local/lib/sgml/%o/%c/%d’, the resulting file name is
/usr/local/lib/sgml/ISO_8879:1986/entities/Added_Latin_1
Note: blanks are transliterated to ‘_’ (and also ‘/’ to ‘%’) and the text class is down cased.
This is a list of catalog entry files.
The files are in the format defined in the SGML Open Draft Technical
Resolution on Entity Management. The Emacs variable is initialized from
the environment variable SGML_CATALOG_FILES or if this variable
is undefined the default is
("CATALOG" "/usr/local/lib/sgml/CATALOG")
A list of SGML entity catalogs to be searched first when parsing the
buffer. This is used in addition to sgml-catalog-files, and
sgml-public-map. This variable is automatically local to the
buffer.
If nil, PSGML will look up external entities by searching the
catalogs in sgml-local-catalogs and sgml-catalog-files and
only if the entity is not found in the catalogs will a given system
identifier be used. If the variable is non-nil and a system identifier is
given, the system identifier will be used for the entity. If no system
identifier is given the catalogs will searched.
This should be a list of file name templates. This variable is
initialized from the environment variable SGML_PATH. This is
the same environment variable that sgmls uses. If the
environment variable is undefined the default is
("%S" "/usr/local/lib/sgml/%o/%c/%d")