Zermelo initially had doubts about the Replacement Axiom (see the letter to Fraenkel from 1922 published in Ebbinghaus 2007: 137), but he eventually accepted it, and a form of it was included in his new axiomatisation published in 1930 (Zermelo 1930). Skolem's formulation is the one usually adopted, though it should be noted that von Neumann's own formulation is rather different and indeed stronger.[36]