**Description: **This syntax construction
states that a variable , which has been
declared to be a set variable by $f statement vx, is also a class
expression. See comments in set.mm for justification.
While it is tempting and perhaps occasionally useful to view cv 1323 as a
"type conversion" from a set variable to a class variable, keep
in mind
that cv 1323 is intrinsically no different from any other
class-building
syntax.
(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 1325 of
predicate
calculus from the wceq 1324 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.) |