|Description: The expression
CHOICE will be used as a readable shorthand for any
form of the axiom of choice; all concrete forms are long, cryptic, have
dummy variables, or all three, making it useful to have a short name.
Similar to the Axiom of Choice (first form) of [Enderton] p. 49.
There is a slight problem with taking the exact form of ax-ac 7969 as our
definition, because the equivalence to more standard forms (dfac2 7641)
requires the Axiom of Regularity, which we often try to avoid. Thus we
take the first of the "textbook forms" as the definition and
form of ax-ac 7969 itself as dfac0 7643. (Contributed by Mario Carneiro,