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We give proof of this theorem below.Although the reader may find the proof entirely acceptable, it nevertheless has a deficiency: it uses an assumption which,however plausible, does not follow from the axioms that we have introduced so far. For that reason it is necessaryto introduce an additional axiom.

Let A be the set of all those a=)W for which w(a) is isomorphic to some ordinal number.As no two distinct ordinals can be isomorphic (one is an initial segment of the other