from
versions.txt:
26/03/95 - mIRC v2.8c etc...
15.Can now also do $$?1 or $?1 which means, try to fill this
value with parameter one if it exists. If parameter one doesnt exist,
ask for it.
That is, it's like:
$iif($1 != $null,$1,$?)
I have to assume that this is deprecated since it isn't in the mirc.hlp, but it still works, at least for now.