  The effect of noise on a quantum system can be described by a set of
operators obtained from the interaction Hamiltonian. Recently it has been shown
that generalized quantum error correcting codes can be derived by studying the
algebra of this set of operators. This led to the discovery of noiseless
subsystems. They are described by a set of operators obtained from the
commutant of the noise generators. In this paper we derive a general method to
compute the structure of this commutant in the case of unital noise.
