 GEM is a powerful theory, and it was meant to be so by its nominalistic forerunners, who were thinking of mereology as a good alternative to set theory. It is also decidable (Tsai 2013a), whereas for example, M, MM, and EM, and many extensions thereof turn out to be undecidable. (For a comprehensive picture of decidability in mereology, see also Tsai 2009, 2011, 2013b.) Just how powerful is GEM? To answer this question, let us focus on the classical formulation based on (P.153) and consider the following generalized sum operator: