Let A[i, j] be zero based on Bezout's identity

           [ii ij]
           [ji jj] is a (k,k) minor of original 3x3 matrix.

def _set_zero(self, i, j, a, b, r, s, t):
        """Let A[i, j] be zero based on Bezout's identity

           [ii ij]
           [ji jj] is a (k,k) minor of original 3x3 matrix.

        """

        L = np.eye(3, dtype='intc')
        L[i, i] = s
        L[i, j] = t
        L[j, i] = -b // r
        L[j, j] = a // r
        self._L.append(L.copy())
        self._A = np.dot(L, self._A)