't Hooft's Polygon Approach Hyperbolically Revisited
  The initial data in the polygon approach to (2+1)D gravity coupled to point particles are constrained by the vertex equations and the particle equations. We establish the hyperbolic nature of the vertex equations and derive some consequences. In particular we are able to identify the hyperbolic group of motions as discrete analogues of the diffeomorphisms in the continuum theory. We show that particles can be included ``hyperbolically'' as well, but they spoil the gauge invariance. Finally we derive consistent sets of initial data. 
Helia R. Hollmann, Ruth M. Williams (Cambridge U., DAMTP)