From: http://alpine.x10host.com/alpine/info/rules.html
Upstream: constitutes upstream source, delivered in non-tarball form

This patch allows you to create specific hooks, so that operations that would normally be global in nature can be customized on a per message/folder/person
basis.

[...]

[For example:]

  * Compose Rules
    This option can be used to generate signatures in cases when setting up a
    role does not work. Examples are

        _TO_ >> {Peter Flinstones} => _SIGNATURE_{~/.petersignature}
        _COLLECTION_ >> {Mail} => _SIGNATURE_{~/ll}