satisfied by that model.</p>
<h3><a name="Theory"></a>Theory</h3>
<p>An IFF <span style="font-style: italic;">theory </span>extends a
type language with a set of expressions
called axioms. These embody a formal or axiomatic semantics. A theory
logically entails an expression of its base language when every model
of the theory is also a model of the expression. Obviously, all axioms
are entailed by a theory. The closure of a theory is the theory with
the same base whose axioms consist of all expressions enta