Discrete Symmetries from Broken $SU(N)$ and the MSSM
  In order that discrete symmetries should not be violated by gravitational effects, it is necessary to gauge them. In this paper we discuss the gauging of $\Z_N$ from the breaking of a high energy $SU(N)$ gauge symmetry, and derive consistency conditions for the resulting discrete symmetry fr om the requirement of anomaly cancellation in the parent symmetry. These results are then applied to a detailed analysis of the possible discrete symmetries forbidding proton decay in the minimal supersymmetric standard model. 
P.L.White