[ 1 ] International Organization for
Standardization, ISO 8879: Information processing---Text and
office systems---Standard Generalized Markup Language (SGML),
([Geneva]: ISO, 1986).
[ 2 ] Work is currently
going on in the standards community to create (using SGML syntax) a
definition of a standard document style semantics and specification
language or DSSSL.
[ 3 ] The actual characters used
for the delimiting characters (the angle brackets, exclamation mark and
solidus) may be redefined, but it is conventional to use the characters
used in this description.
[ 4 ] The example is taken
from William Blake's Songs of innocence and experience (1794). The
markup is designed for illustrative purposes and is not TEI-conformant.
[ 5 ] Note that this simple example has not addressed
the problem of marking elements such as sentences explicitly; the
implications of this are discussed below in section 6.
[ 6 ] Like the delimiters, these are assigned formal names
by the standard and may be redefined with an appropriate SGML
declaration. [ 7 ] What are here called group
connectors are referred to by the SGML standard simply as connectors;
the longer term is preferred here to stress the fact that these
connectors are used only in SGML model groups and name groups. Like
the delimiters and the occurrence indicators, group connectors are
assigned formal names by the standard and may be redefined with an
appropriate SGML declaration. [ 8 ] It will not have
escaped the astute reader that the fact that verse paragraphs need not
start on a line boundary seriously complicates the issue; see further
section 6.
[ 9 ] By convention case is significant in
entity names, unlike element names. [ 10 ] Strictly
speaking, SGML does not require system entities to be files; they can
in principle be any data source available to the SGML processor:
files, results of database queries, results of calls to system
functions --- anything at all. It is simpler, however, when first
learning SGML, to think of system entities as referring to files, and
this discussion therefore ignores the other possibilities. All
existing SGML processors do support the use of system entities to refer
to files; fewer support the other possible uses of system entities.