  Recent exact predictions for the massive scaling limit of the two dimensional
XY-model are based on the equivalence with the sine-Gordon theory and include
detailed results on the finite size behavior. The so-called step-scaling
function of the mass gap is simulated with very high precision and found
consistent with analytic results in the continuum limit. To come to this
conclusion, an also predicted form of a logarithmic decay of lattice artifacts
was essential to use for the extrapolation.
