  The paper is concerned with a posteriori error bounds for a wide class of
numerical schemes, for $n\times n$ hyperbolic conservation laws in one space
dimension. These estimates are achieved by a "post-processing algorithm",
checking that the numerical solution retains small total variation, and
computing its oscillation on suitable subdomains. The results apply, in
particular, to solutions obtained by the Godunov or the Lax-Friedrichs scheme,
backward Euler approximations, and the method of periodic smoothing. Some
numerical implementations are presented.
