as a semi pointless aside on a list where most people think procmail is an
abomination, the second ':' in ':0:' means 'lock the destination file
before delivering'. So you're protecting /dev/null from becoming
corrupted =). Of course in this case you're just testing so it doesn't
matter, but at least in older versions of procmail using :0: instead of :0
when delivering to remote email addresses or a pipe would still lock
something, creating an unnecessary bottleneck
