  We employ the simplest possible models of scalar-fermion interactions that
are consistent with the gauge symmetries of the Standard Model and permit no
proton decay to analyze the connections possible among processes that break
baryon number by two units. In this context we show how the observation of
$n$-${\bar n}$ oscillations and of a pattern of particular nucleon-antinucleon
conversion processes --- all accessible through e-d scattering --- namely,
selecting from $e^- p \to e^+ {\bar p}$, $e^- p \to {\bar n} {\bar \nu}$, $e^-
n \to {\bar p} {\bar \nu}$, and $e^- n \to e^- {\bar n} $ would reveal that the
decay $\pi^- \pi^- \to e^- e^-$ must occur also. This latter process is the
leading contribution to neutrinoless double beta decay in nuclei mediated by
new short-distance physics, in contrast to that mediated by light Majorana
neutrino exchange. The inferred existence of $\pi^- \pi^- \to e^- e^-$ would
also reveal the Majorana nature of the neutrino, though the absence of this
inference would not preclude it.
