Juan de Lara Roswitha Bardohl Hartmut Ehrig Karsten Ehrig Ulrike Prange Gabriele Taentzer 139-163 2007 376 Theor. Comput. Sci. 3 http://dx.doi.org/10.1016/j.tcs.2007.02.001 db/journals/tcs/tcs376.html#LaraBEEPT07