This axiom implies ZFC's axiom of choice since for every set formula_4 of nonempty sets, formula_301 (the restriction of formula_1 to formula_4) is a choice function for formula_304 In 1964, William B. Easton proved that global choice is stronger than the axiom of choice by using forcing to construct a model that satisfies the axiom of choice and all the axioms of NBG except the axiom of global choice.