colon-zero   ...   what i learned today , unix
thu 2004-apr-29 20:20:35 pdt   ...   permalink

From the procmailrc man page:

In the old days, the `:0' that marks the beginning of a recipe, had to be changed to `:n', whereby `n' denotes the number of conditions that follow.

Gotta love stuff that's just there for historical reasons.