  A recent publication reports that heavy-ion fusion cross sections at extreme
subbarrier energies show a continuous change of their logarithmic slope with
decreasing energy, resulting in a much steeper excitation function compared
with theoretical predictions. We show that the energy dependence of this slope
is partly due to the asymmetric shape of the Coulomb barrier, that is its
deviation from a harmonic shape. We also point out that the large low-energy
slope is consistent with the surprisingly large surface diffusenesses required
to fit recent high-precision fusion data.
