  A proof of critical conformal invariance of Green's functions for a quite
wide class of models possessing critical scale invariance is given. A simple
method for establishing critical conformal invariance of a composite operator,
which has a certain critical dimension, is also presented. The method is
illustrated with the example of the Gross--Neveu model and the exponents \et\
at order $1/n^3$, \Dl\ and $1/\nu$ at order $1/n^2$ are calculated with the
conformal bootstrap method.
