all the outer fermions being left-handed. In this case, the quantum numbers of the internal fermion $\psi$ are equal to those of a (light) neutrino, and the mass term is picked out from the propagator.

Other examples can be identified as easily. To list a few more, the afore-mentioned $W_R-N-W_R$ exchange diagram is also contained in T-I-1-i with vectors ($W_{R}^{\pm}$), all outer fermions now being right-handed. Since $N$ must be heavy, however, this is now a short-range contribution. Concrete models can lead to the occurrence of more than one of the operators listed, an example is provided by (trilinear) R-parity violating (RPV) supersymmetry (SUSY). The six short-range diagrams [@Hirsch:1995zi; @Hirsch:1995ek] for the RPV SUSY mechanism of [$0\nu\beta\beta$ ]{}decay [@Mohapatra:1986su], correspond to class SFS and are identified as T-I-1-i (${\tilde e}-\tilde{\chi}^{0}-{\tilde e}$ diagram), T-I-2-i-b (${\tilde e}-\tilde{\chi}^{0}-{\tilde d}$), T-I-2-ii-b (${\tilde e}-\tilde{\chi}^{0}-{\tilde u}$), T-I-2-iii-a (${\tilde u}-\tilde{\chi}^{0}/\tilde{g}-{\tilde d}$), T-I-4-i (${\tilde u}-\tilde{\chi}^{0}/\tilde{g}-{\tilde u}$) and T-I-5-i (${\tilde d}-\tilde{\chi}^{0}/\tilde{g}-{\tilde d}$). In those diagrams, the neutralino $\tilde{\chi}^{0}$ corresponds to $\psi(0,{\bf 1})$, while the gluino $\tilde{g}$ corresponds to $\psi(0,{\bf 8})$. Finally, the leptoquark (LQ) mechanism of [@Hirsch:1996qy; @Hirsch:1996ye] is a long-range contribution of the class SFV with T-I-2-i-b and T-I-2-ii-b. The internal fermion $\psi(0,{\bf 1})$ is again identified with 