Anne-Cécile Caron Jean-Luc Coquidé Decidability of Reachability for Disjoint Union of Term Rewriting Systems. 31-52 1994 126 Theor. Comput. Sci. 1 db/journals/tcs/tcs126.html#CaronC94 http://dx.doi.org/10.1016/0304-3975(94)90267-4