Where is setuser ?

[Irwan Hadi]

        ulimit -v 2048

        csh -cf '/var/qmail/rc &'

        supervise /var/lock/qmail-smtpd tcpserver -v -x/etc/tcp.smtp.cdb
-u$USERID -g$GROUPID 0 25 \
        rblsmtpd qmail-smtpd 2>&1 | setuser qmaill accustamp | \
        setuser qmaill cyclog -s5000000 -n5 /var/log/qmail/qmail-smtpd &

I want to run this script from my init 
My question is, where could I found "setuser" command ? because W/O this,
the script won't work, and I must do manually the command.
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