QM and Universe both allow the user to define their own
user exits that run subroutines using the ICONV() and
OCONV() functions. In both systems, the user exit must
start with a "U" while Universe further limits the length of
the subroutine name to 5 characters. I don't think there
needs to be any limitation myself - the PICK principle is
surely to allow the programmer to do just about anything
(s)he wants to.
The subroutine would generally have the calling
variables - the returned data, the passed data, an
ICONV/OCONV signal flag, and an error signal
(@SYSTEM.RETURN.CODE or an equivalent?). I think the
returned data ought to be the first variable. This is the
protocol used for Universe I-type SUBR subroutines and I
find it useful for allowing multiple uses of subroutines.
The order of the rest of the variables is not that
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