| 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 1616
of
predicate calculus in terms of the wceq 1615
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 1616 or wceq 1615, 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 2163 for more information on the set
theory usage of wceq 1615.) |
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