  This is an extended version of the Comment that contains the criticism
against an applicability of the diffusive transport model for the explanation
of the ring-shaped spatial pattern formation in the exciton photoluminescence
for the GaAs/AlGaAs double quantum wells [Phys. Rev. Lett. 92, 117404 (2004)].
The model has been used in other similar papers published in the PRL and the
PRB afterwards.
