Michael A. Harrison Ivan M. Havel Amiram Yehudai On Equivalence of Grammars Through Transformation Trees. 173-205 1979 9 Theor. Comput. Sci. db/journals/tcs/tcs9.html#HarrisonHY79 http://dx.doi.org/10.1016/0304-3975(79)90024-0