  The goal of the present paper is to study the weak--strong uniqueness problem
for the compressible Navier--Stokes system with a general barotropic pressure
law. Our results include the case of a hard sphere pressure of Van der Waals
type with a non--monotone perturbation and a Lipschitz perturbation of a
monotone pressure. Although the main tool is the relative energy inequality,
the results are conditioned by the presence of viscosity and do not seem
extendable to the Euler system.
