Why "Anomalous" $J/\psi$ Suppression in $Pb + Pb$ Collisions Signals High-density Parton Matter

We argue that the A-dependence of absorption of J/\psi by (partonic) comovers is steeper than assumed in most phenomenological models, because the absorption process is dominated by quasi-perturbative QCD interactions. Our argument is supported by results recently obtained in the framework of the parton cascade model. We predict significant ``anomalous'' suppression for Pb+Pb collisions at the CERN-SPS, but not for S+U collisions.