  We have investigated parametric seeding of a microresonator frequency comb
(microcomb) by way of a pump laser with two electro-optic-modulation sidebands.
We show that the pump-sideband spacing is precisely replicated throughout the
microcomb's optical spectrum, and we demonstrate a record absolute line-spacing
stability for microcombs of $1.6\times10^{-13}$ at 1 s. The spectrum of a
parametric comb is complex, and often non-equidistant subcombs are observed.
Our results demonstrate that parametric seeding can not only control the
subcombs, but can lead to the generation of a strictly equidistant microcomb
spectrum.
