Grothendieck rings of theories of modules

The model-theoretic Grothendieck ring of a first order structure, as defined by Krajicěk and Scanlon, captures some combinatorial properties of the definable subsets of finite powers of the structure. In this paper we compute the Grothendieck ring, K0(MR)K0(MR), of a right RR-module M  , where RR is any unital ring. As a corollary we prove a conjecture of Prest that K0(MR)K0(MR) is non-trivial, whenever M is non-zero. The main proof uses various techniques from simplicial homology and lattice theory to construct certain counting functions.