  Richardson extrapolation is applied to a simple first-order upwind difference
scheme for the approximation of solutions of singularly perturbed
convection-diffusion problems in one dimension. Robust a posteriori error
bounds are derived for the proposed method on arbitrary meshes. It is shown
that the resulting error estimator can be used to stear an adaptive mesh
algorithm that generates meshes resolving layers and singularities. Numerical
results are presented that illustrate the theoretical findings.
