def METHOD_NAME(self, fields):
    M12, M11, U, phi = self._updated(fields)
    P12 = M12.propagator(self.inverter)
    P11 = M11.propagator(self.inverter)
    m1 = self.m1
    m2 = self.m2
    def _mat(dst, src):
        dst @= (
            src
            + (m2 - m1) * Pplus * g.gamma[5] * P12 * Pplus * src
            + (m1 - m2) * Pminus * g.gamma[5] * P11 * Pminus * src
        )
    return g.matrix_operator(_mat)