This means that in an extensional mereology ‘O’ could be used as a primitive and ‘P’ defined accordingly, as in Goodman (1951), and it can be checked that the theory defined by postulating (61) together with the Unrestricted Sum principle (P.153) and the Antisymmetry axiom (P.3) is equivalent to GEM (Eberle 1967). Another elegant axiomatization of GEM, due to an earlier work of Tarski (1929),[23] is obtained by taking just the Transitivity axiom (P.2) together with the Sum2-analogue of the Uni