  A certain generalisation of the hierarchy of mKdV equations (modified KdV),
which forms an integrable system, is studied here. This generalisation is based
on a Lax operator associated to the equations, with principal components of
degrees between -3 and 0. The results are the following ones: 1) an isomorphism
between the space of jets of the system and a quotient of ${Sl}_2({\CC}((t)))$;
2) the fact that the monodromy matrixes of the Lax operators have, morover,
Poisson brackets given by the trigonometric r-matrix; 3) a definition of the
action of screening operators on the densities; 4) an identification of the
intersection of the kernel with the integrals of motion.
