  A highly degenerate family of states [proposed in PRB 63, 134503 (2001)] is
proven to really minimize the Hamiltonian of the fully frustrated XY model on a
dice lattice. The harmonic fluctuations are shown to be no consequence for the
removal of the accidental degeneracy of these states, so a particular vortex
pattern can be stabilized only by the anharmonic fluctuations. The structure of
this pattern is found and the temperature of its disordering due to the
proliferation of domain walls is estimated. The extreme smallness of the
fluctuations induced free energy of domain walls leads to the anomalous
prominence of the finite-size effects, which prevent the observation of
vortex-pattern ordering in numerical simulations. In such a situation the loss
of phase coherence may be related to the dissociation of fractional vortices
with topological charges 1/8. In a physical situation the magnetic interaction
of currents in a Josephson junction array will be a more important source for
the stabilization of a particular vortex pattern than the anharmonic
fluctuations.
