Description: The expression will be used as a readable shorthand for
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-ac8860 as our
definition, because the equivalence to more standard forms (dfac28532)
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-ac8860 itself as dfac08534. (Contributed by Mario Carneiro,