  Enhancement and suppression of nonlinear processes in coupled systems of
plasmonic converters and quantum emitters are well-studied theoretically,
numerically and experimentally, in the past decade. Here, in difference, we
explicitly demonstrate --with a single equation-- how the presence of a Fano
resonance leads to cancellation of nonresonant terms in a four-wave mixing
process. Cancellation in the denominator gives rise to enhancement in the
nonlinearity. The explicit demonstration, we present here, guides us to the
method for achieving more and more orders of magnitude enhancement factors via
path interference effects. We also study the coupled system of a plasmonic
converter with two quantum emitters. We show that the potential for the
enhancement increases dramatically due to better cancellation of the terms in
the denominator.
