  The key to membrane theory is to enlarge the diffeomorphism group until 4D
gravity becomes almost topological. Just one ghost survives and its central
charges can cancel against matter. A simple bosonic membrane emerges, but its
flat D = 28 target space is unstable. Adding supersymmetry ought to give
calculable (2,2) membranes in 12 target dimensions, but (2,1) membranes won't
work.
