@ARTICLE{DasMit_j1997_c, author = {J{\"u}rgen~Dassow and Victor~Mitrana}, title = {Cooperation in context-free grammars}, journal = tcs, year = 1997, volume = 180, pages = {353--361}, url = {http://dx.doi.org/10.1016/S0304-3975(96)00330-1} }