**Description: **Extend wff definition to
include class equality.
For a general discussion of the theory of classes, see
http://us.metamath.org/mpeuni/mmset.html#class.
(The purpose of introducing wff 𝐴 = 𝐵 here, and not in set theory
where it belongs, is to allow us to express i.e. "prove" the
weq 1392
of
predicate calculus in terms of the wceq 1243
of set theory, so that we
don't "overload" the = connective with
two syntax definitions. This
is done to prevent ambiguity that would complicate some Metamath
parsers. For example, some parsers - although not the Metamath program
- stumble on the fact that the = in 𝑥 = 𝑦 could be the = of
either weq 1392 or wceq 1243, although mathematically it makes no
difference.
The class variables 𝐴 and 𝐵 are introduced
temporarily for the
purpose of this definition but otherwise not used in predicate calculus.
See df-cleq 2033 for more information on the set theory usage
of
wceq 1243.) |