Example in procmailex man page

Hi, the procmailex man page gives the following example, which I am
considering using.  However, I have one question about it (indicated
below):

    If you are fairly new to procmail and plan to experiment a little
    bit it  often helps  to  have  a  safety  net of some sort.
    Inserting the following two recipes above all other recipes will
    make sure that of all arriving mail always  the  last 32  messages
    will be preserved.  In order for it to work as intended, you have to
    create a directory named `backup' in  $MAILDIR  prior  to  inserting
    these  two recipes.
    
            :0 c
            backup
    
            :0 ic
            | cd backup && rm -f dummy `ls -t msg.* | sed -e 1,32d`
                                 ^^^^^

What I want to know is the purpose of the dummy.  Thanks.

Andrew.

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