**Description: **This syntax construction
states that a variable , which has been
declared to be a setvar variable by $f statement vx, is also a class
expression. This can be justified informally as follows. We know that
the class builder is a class by cab 2126.
Since (when
is distinct from
) we have by
cvjust 2135, we can argue that the syntax " " can be viewed as
an abbreviation for " ". See the
discussion
under the definition of class in [Jech] p.
4 showing that "Every set can
be considered to be a class."
While it is tempting and perhaps occasionally useful to view cv 1331 as a
"type conversion" from a setvar variable to a class variable,
keep in
mind that cv 1331 is intrinsically no different from any other
class-building syntax such as cab 2126, cun 3075,
or c0 3369.
For a general discussion of the theory of classes and the role of cv 1331,
see https://us.metamath.org/mpeuni/mmset.html#class 1331.
(The description above applies to set theory, not predicate calculus.
The purpose of introducing here, and not in set theory where
it belongs, is to allow us to express i.e. "prove" the weq 1480 of
predicate calculus from the wceq 1332 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.) |