Text within INCLUDE and IGNORE marked sections is
parsed or skipped by the parser, as appropriate. They are not
often used outside of DTDs, but the IGNORE marked section
is occasionally useful as a way of `commenting out' large sections of
text which might itself include SGML comments <!--
... -->.
Sometimes, it is necessary to have parts of your document which
vary depending on whether the document is to be viewed on paper or
online. In general, the best way to manage this is using the span element (see Section 4.11), but this
is limited to use within paragraphs. If you need to
conditionally include higher-level chunks of material, you will have
to resort to the blunter instrument of a marked section. We may add a
suitable element to the DTD in future - probably called div -
and deprecate this method.
The Starlink general DTD defines two parameter entities
%Only.paper and %Only.web, which you can
use to include or ignore text in a marked section. By default, both
of these are set to the string
`IGNORE', but when you use the sgml2docs script (see
Section 6.2), one or the other of these will be set to `INCLUDE', as
appropriate for the type of file being generated.
You would use this facility as follows:
<p>Here is some text. <![ %Only.web [ And here is some which would appear only in webpages. ]]>
It is, I hope, clear that you could get yourself in a terrible mess here, and create a document which is only valid for some settings of the entities. Please use this feature with restraint: take care that your document remains valid for all settings, including the case where both entities are set to `IGNORE'.