**Description: **Extend wff definition to
include class equality.
For a general discussion of the theory of classes, see
http://us.metamath.org/mpegif/mmset.html#class.
(The purpose of introducing
here, and not in set theory
where it belongs, is to allow us to express i.e. "prove" the
weq 1620
of
predicate calculus in terms of the wceq 1619
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
could be the of
either weq 1620 or wceq 1619, although mathematically it makes no
difference. The class variables and are introduced
temporarily for the purpose of this definition but otherwise not used in
predicate calculus. See df-cleq 2249 for more information on the set
theory usage of wceq 1619.) |