=head2 crosspost

Limits crossposting and followups.  This module takes, in some ways, the
opposite approach from the newsgroups module; rather than listing
newsgroups which are allowed, it allows one to set a limit on the number
of groups crossposted, on the number of groups to which followups would
go, and on which groups can be crossposted to or to which followups can be
directed.

A group can be excluded from the Newsgroups header or from the Followup-To
header in one of two ways; either messages posted to that group or
messages that direct followups to a set of newsgroups including that group
can be rejected, or that group can be silently removed from the Newsgroups
and/or Followup-To headers.  The latter behavior is generally not
recommended for robomoderators, since moderators are generally discouraged
to make decisions about where posts should be posted for the poster.
Returning the post and asking the poster to choose a more appropriate set
of groups is generally a better approach.

This module will remove a Followup-To header (and not add one) if its
content is/would be identical to the Newsgroups header.

This module takes the following configuration directives:

=over 4

=item crosspost max COUNT

Reject any message crossposted to more than C<COUNT> newsgroups.

=item crosspost remove|reject GROUP

Either remove C<GROUP> silently from the Newsgroups header if present or
reject all articles crossposted to C<GROUP>.

=item followup max COUNT

Reject any messages that would direct followups to more than C<COUNT>
newsgroups, bearing in mind that if no Followup-To header is present,
followups would go to all groups in the Newsgroups header.

=item followup remove|reject GROUP

Either remove C<GROUP> silently from the followups if present or reject
all articles crossposted to C<GROUP>.

=back

This module may return one of the following failure messages:

=over 4

=item Invalid crosspost to %s

The message was crossposted to a group mentioned in a C<crosspost reject>
directive.

=item Followups would go to %s

Followups to the message would go to a group mentioned in a C<followup
reject> directive.

=item Excessively crossposted

The message was crossposted to more groups than allowed by a C<crosspost
max> directive.

=item Followups would go to too many groups

Followups to the message would go to more groups than allowed by a
C<followup max> directive.

=back

This module works with a Newsgroups header, so if other modules need to be
applied to construct the Newsgroups header, they should run before this
one.  This module takes no arguments.