def _find_error_evaluator_fast(self, synd, sigma, k=None):
        '''Compute the error (or erasures if you supply sigma=erasures locator polynomial) evaluator polynomial Omega from the syndrome and the error/erasures/errata locator Sigma. Omega is already computed at the same time as Sigma inside the Berlekamp-Massey implemented above, but in case you modify Sigma, you can recompute Omega afterwards using this method, or just ensure that Omega computed by BM is correct given Sigma (as long as syndrome and sigma are correct, omega will be correct).'''
        n = self.n
        if not k: k = self.k

        # Omega(x) = [ Synd(x) * Error_loc(x) ] mod x^(n-k+1) -- From Blahut, Algebraic codes for data transmission, 2003
        return (synd * sigma)._gffastmod(Polynomial([GF2int(1)] + [GF2int(0)] * (n-k+1)))
Compute the error (or erasures if you supply sigma=erasures locator polynomial) evaluator polynomial Omega from the syndrome and the error/erasures/errata locator Sigma. Omega is already computed at the same time as Sigma inside the Berlekamp-Massey implemented above, but in case you modify Sigma, you can recompute Omega afterwards using this method, or just ensure that Omega computed by BM is correct given Sigma (as long as syndrome and sigma are correct, omega will be correct).