procmailrc syntax

Although I used procmail for so long I never figure out what kind of =

"script" rules it follows.  For example in one of my rc script I try to =

calculate a list file ("dist"):

   DISTCOUNT=3D`grep -cv '^(' dist` -1

the calculation part fails, end up in the log:

   procmail: Skipped "-1"

How can I get over this without getting it to external script for a simple =

calculation like this?

Zhiliang