IFPDOn[exp_, q1, q2, ...] changes from
FeynAmpDenominator[...] representation to the
IFPD one (Inverse Feynman Propagator Denominator). I.e.,
FeynAmpDenominator[PropagatorDenominator[a,b]] is replaced
by 1/IFPD[a,b] and the q1, q2, ... are the
integration momenta.