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This macro invokes the procedure registered to display
the usage text. Normally, this will be
optionUsage from the
AutoOpts library, but you may select another procedure by specifying
usage = "proc_name" program attribute. This procedure must
take two arguments first, a pointer to the option descriptor, and
second the exit code. The macro supplies the option descriptor
automatically. This routine is expected to call
the provided exit code.
optionUsage routine also behaves differently depending
on the exit code. If the exit code is zero, it is assumed that
assistance has been requested. Consequently, a little more
information is provided than when displaying usage and exiting
with a non-zero exit code.