  The coupling of gravity to dust helps to discover simple quadratic
combinations of the gravitational super-Hamiltonian and supermomentum whose
Poisson brackets strongly vanish. This leads to a new form of vacuum
constraints which generate a true Lie algebra. We show that the coupling of
gravity to a massless scalar field leads to yet another set of constraints with
the same property, albeit not as simple as that based on the coupling to dust.
