texinfo: Conditional Nesting

Conditionals

Testing for Texinfo Commands

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18.7 Conditional Nesting
========================

Conditionals can be nested; however, the details are a little tricky.
The difficulty comes with failing conditionals, such as '@ifhtml' when
HTML is not being produced, where the included text is to be ignored.
However, it is not to be _completely_ ignored, since it is useful to
have one '@ifset' inside another, for example--that is a way to include
text only if two conditions are met.  Here's an example:

     @ifset somevar
     @ifset anothervar
     Both somevar and anothervar are set.
     @end ifset
     @ifclear anothervar
     Somevar is set, anothervar is not.
     @end ifclear
     @end ifset

  Technically, Texinfo requires that for a failing conditional, the
ignored text must be properly nested with respect to that failing
conditional.  Unfortunately, it's not always feasible to check that
_all_ conditionals are properly nested, because then the processors
could have to fully interpret the ignored text, which defeats the
purpose of the command.  Here's an example illustrating these rules:

     @ifset a
     @ifset b
     @ifclear ok  - ok, ignored
     @end junky   - ok, ignored
     @end ifset
     @c WRONG - missing @end ifset.

  Finally, as mentioned above, all conditional commands must be on lines
by themselves, with no text (even spaces) before or after.  Otherwise,
the processors cannot reliably determine which commands to consider for
nesting purposes.