|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 2042.
is distinct from
) we have by
cvjust 2051, we can argue that the syntax " " can be viewed as
an abbreviation for " ". See the
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 1258 as a
"type conversion" from a setvar variable to a class variable,
mind that cv 1258 is intrinsically no different from any other
class-building syntax such as cab 2042, cun 2943,
or c0 3252.
For a general discussion of the theory of classes and the role of cv 1258,
(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 1408 of
predicate calculus from the wceq 1259 of set theory, so that we don't
connective with two syntax definitions. This is done
to prevent ambiguity that would complicate some Metamath