nt, in the
                sense that small perturbations in A can make one or
                more of the eigenvalues lie outside the unit circle;
                perturbed values were used to solve the equation
                (but the matrix A is unchanged);
          = 2:  if matrix A is not stable (that is, one or more of
                the eigenvalues of A has a non-negative real part),
                if DISCR = .FALSE., or not convergent (that is, one
                or more of the eigenvalues of A