| Description: Extend wff definition to
include class equality.
(The purpose of introducing  
  here, and not in set theory
where it belongs, is to allow us to express i.e. "prove" the
weq 957
of
predicate calculus in terms of the wceq 956
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 957 or
wceq 956, 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 1469 for more information on the set
theory usage of wceq 956.) |
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