  It is a step in the proof of the stabilization of the twisted trace formula.
We generalize to the twisted case the proposition 3.1 of the third Arthur's
paper on the stabilization. That is, consider the difference between an
omega-equivariant weighted orbital integral (relative to a Levi subspace of a
twisted space) and its endoscopic avatar. Then this difference is the ordinary
omega-orbital integral of some function on the Levi subspace. Here, the
base-field is non-archimedean.
