  Starting from first principles, we formulate a theory of wave packet
propagation in a nonlinear, disordered medium of any dimension, through the
derivation of a Fokker-Planck transport equation. Our theory is based on a
diagrammatic expansion of the wave packet's density, and is supported by a
heuristic picture that involves a Boltzmann equation with an effective,
external potential. Our approach also confirms results obtained in previous
work for two-dimensional, nonlinear disordered media.
