Aleksandrs Belovs Ansis Rosmanis Juris Smotrovs Multi-letter Reversible and Quantum Finite Automata. 60-71 2007 conf/dlt/2007 Developments in Language Theory http://dx.doi.org/10.1007/978-3-540-73208-2_9 db/conf/dlt/dlt2007.html#BelovsRS07