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Mirrors > Home > MPE Home > Th. List > dfcleq  Unicode version 
Description: Define the equality
connective between classes. Definition 2.7 of
[Quine] p. 18. Also Definition 4.5 of
[TakeutiZaring] p. 13; Chapter 4
provides its justification and methods for eliminating it. Note that
its elimination will not necessarily result in a single wff in the
original language but possibly a "scheme" of wffs.
This is an example of a somewhat "risky" definition, meaning
that it has
a more complex than usual soundness justification (outside of Metamath),
because it "overloads" or reuses the existing equality symbol
rather
than introducing a new symbol. This allows us to make statements that
may not hold for the original symbol. For example, it permits us to
deduce
We could avoid this complication by introducing a new symbol, say
=_{2},
in place of However, to conform to literature usage, we retain this overloaded definition. This also makes some proofs shorter and probably easier to read, without the constant switching between two kinds of equality. See also comments under dfclab 2468, dfclel 2477, and abeq2 2586. In the form of dfcleq 2475, this is called the "axiom of extensionality" by [Levy] p. 338, who treats the theory of classes as an extralogical extension to our logic and set theory axioms. For a general discussion of the theory of classes, see http://us.metamath.org/mpeuni/mmset.html#class. (Contributed by NM, 15Sep1993.) 
Ref  Expression 

dfcleq.1 
Ref  Expression 

dfcleq 
Step  Hyp  Ref  Expression 

1  cA  . . 3  
2  cB  . . 3  
3  1, 2  wceq 1662  . 2 
4  vx  . . . . . 6  
5  4  cv 1661  . . . . 5 
6  5, 1  wcel 1724  . . . 4 
7  5, 2  wcel 1724  . . . 4 
8  6, 7  wb 178  . . 3 
9  8, 4  wal 1556  . 2 
10  3, 9  wb 178  1 
Colors of variables: wff set class 
This definition is referenced by: dfcleq 2475 
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