Jari's pm-jalist.rc mailing list identifier

I am trying getting to get Jari Aalto's pm-jalist.rc to allow
non-mailing-list mail to "fall off the end".

Here is what I have so far (note that this is the last thing in my
.procmailrc):

    PMSRC = /usr/local/share/procmail-lib/lib

    INCLUDERC = /usr/local/share/procmail-lib/lib/pm-jalist.rc

    :0 :                        # if list name was grabbed
    * ! LIST ?? ^^^^    # does this mean "if LIST is non-null"?
    $LIST.spool

It works great if pm-jalist.rc actually identifies a list (and hence,
LIST has a value like "zsh-users" and gets put into the desired spool
file, that is, zsh-users.spool).  But if the value of LIST is null,
then it gets put into the file .spool instead of falling of the end.

My understanding is that the following condition:

    * ! LIST ?? ^^^^

says "do the action if LIST is non-null", due to the following bit in
"man 5 procmailrc":

       ^^        Anchor the expression at the very start  of  the
                 search area, or if encountered at the end of the
                 expression, anchor it at the  very  end  of  the
                 search area.

so the action should not be called if LIST is null.

Can somebody please help me see the problem here?

Thankyou.

Andrew.
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