  In this paper, we determine a condition of applicability of a very simple
formalism for the calculation of a fast process taking place in a
non-equilibrium medium, in which the particle distribution functions are frozen
in time.
  Indeed, a well known obstacle for the use of such a naive formalism is the
appearance of the so-called ``pinch singularities''. We argue that these
potentially dangerous terms can be regularized, and that they are negligible if
the characteristic time-scale of the process under study is small compared to
the relaxation time of the system.
