  Because of IR (pinch) singularities a resummation is necessary in
non-equilibrium field theories, that can be performed by using Kadanoff--Baym
equations. Taking Landau prescription correctly into account, Kadanoff--Baym
equations reduce to Boltzmann equations only in a restricted kinematical range;
in other cases a new equation (the former constraint equation) has to be
considered. In relaxation time approximation this new equation results in the
shifting and smearing of multiparticle thresholds.
