Trying to use $1 from -a argument

        I'm using the local_procmail mailer in sendmail 8.7.1, which
takes addresses of user+detail and sends the mail to procmail for user
with -a detail.  The procmail man page indicates that the argument to -a
is assigned to $1, which is true.  My problem is, how can I have a
procmailrc rule to take advantage of this?  I've tried

:0:
* 1 ?? .+
mail.${1}.spool

        and

:0:
* ?[ -n "$1" ]
mail.${1}.spool

        and nothing gets filed.  I've tried

:0:
* ?[ -n '$1' ]
mail.${1}.spool

        and

:0:
* ?[ -n $1 ]
mail.${1}.spool

        and every mail gets filed.  How can I create a condition where
_only_ mail with $1 set gets filed?

                Sten

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