[RIF] Horn rule semantics based on the condition language

Hi RIF Colleagues,

this is in response to my action item about Horn rule semantics based on the
condition language, and useful for its extension to the Horn rule language:
http://www.w3.org/2005/rules/wg/wiki/B._Extension%3A_RIF_Horn_Rule_Language

Michael, Jeff, and I have almost finished work on this topic, and Michael
plans to respond to his action item about this soon.

Basically, we have specialized the notions of satisfaction of conditions
to the existential-conjunctive bodies of Horn rules, introduced a compatible
semantics for Horn rule heads, and then combined these for the semantics of
Horn rules.

Best,
Harold