Description: Define the "proves" relation on a set. A wff is true in a model M
if for every valuation s e. ( M ^m _om ) , the interpretation of the
wff using the membership relation on M is true. Since |= is
defined in terms of the interpretations making the given formula true,
it is not defined on the empty "model" M = (/) , since there are no
interpretations. In particular, the empty set on the LHS of |=
should not be interpreted as the empty model. Statement prv0 shows
that our definition yields (/) |= U for all formulas, though of
course the formula E. x x = x is not satisfied on the empty model.
(Contributed by Mario Carneiro, 14-Jul-2013)