The third way to do inference in SNePS is to use rules. Rules
in SNePS are not kept in a separate location from the rest of the
knowledge base, as they are in many production systems. Rather, rules
are propositions that Cassie believes. As such, they can themselves be
reasoned about. But in what follows, we'll just use them as inference
rules in a predicate logic, using an example from
Rich & Knight 1991: 134-135.