procmail block, flow

For incoming mail marked with X-SpamProbe: SPAM
I am saving copies to a compressed mailbox. However, to conserve disk space
I only want to save the first few lines. Here is my recipe:

:0
* ^X-SpamProbe: SPAM
{
:0 fw
| head -n 100

:0 c:
| gzip >> $HOME/spambox.gz
}


The rule which carbon-copies into the spambox works fine, it never fails.
However when I inspect the spambox I find that some, but not all, emails
are truncated to 100 lines as intended.

This is the only rule in all of .procmailrc which writes to spambox. Does
the syntax as I wrote it guarantee that "head -n 100" will always run?

--
Jem Berkes
www.sysdesign.ca