  Stationary spherically symmetric gravity is equivalent to a nonlinear coset
sigma model on SL(2,R)/SO(2) coupled to a gravitational remnant. Classically
there are stationary solutions besides the static Schwarzschild metric labeled
by the Schwarzschild mass $m$ and the Taub-NUT charge $l$. Imposing the SL(2,R)
symmetry at the quantum level the Wheeler-DeWitt equation becomes related to
the Casimir operator on the coset, which makes the system amenable to exact
quantization.
