Sometimes it is useful to invoke more than one call-back function at once. The
"+" call-back function takes two subsequent call-back functions as arguments
and calls each in turn. The return value from the "+" call-back function is the
return from the second call. For an example of the use of the "+" and
FILE_OPENR call-back functions see the description of %aw in
Attach window. Note that the result of
clearwin_string@('CALLBACK_REASON') reflects the original reason for the
call-back and not "+".