Syntax Definition cv 991

Description: This syntax construction states that a variable x, which has been declared to be a set variable by $f statement vx, is also a class expression. This can be justified informally as follows. We know that the class builder {y | y e. x} is a class by cab 1505. Since (when y is distinct from x) we have x = {y | y e. x} by cvjust 1513, we can argue that that the syntax "class x" can be viewed as an abbreviation for "class {y | y 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 cv 991 as a "type conversion" from a set variable to a class variable, keep in mind that cv 991 is intrinsically no different from any other class-building syntax such as cab 1505, cun 2097, or c0 2332.

(The description above applies to set theory, not predicate calculus. The purpose of introducing class x here, and not in set theory where it belongs, is to allow us to express i.e. "prove" the weq 993 of predicate calculus from the wceq 992 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.)

Hypothesis

Ref	Expression
vx	set x

Assertion

Ref	Expression
cv	class x

This syntax is primitive. The first axiom using it is ax-5 996.

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