__ flag causes the flag DEFINED_[function] to be written to
the zcc_opt.def file. This is then
picked up by the startup code and appropriate code is generated to call
this function - NB it won't automatically turn off OZ preservation of the
screen. This __APPFUNC__ call could be applied to other functions to 
enable your code to take over error handling for example, however that
isn't supported at  the moment.</P>

<P>For an example of this see the examples/app/dstar.c demo.</P>

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