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 cab2442. Since (when
is distinct from ) we have by
cvjust2451, we can argue that the syntax "classx " can be
viewed as
an abbreviation for "class{|e.x}". 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 cv1394 as a
"type conversion" from a setvar variable to a class variable,
keep in
mind that cv1394 is intrinsically no different from any other
class-building syntax such as cab2442, cun3473,
or c03784.

(The description above applies to set theory, not predicate calculus.
The purpose of introducing classx here, and not in set theory where
it belongs, is to allow us to express i.e. "prove" the weq1733 of
predicate calculus from the wceq1395 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.)