1. Blaine, L., Goldberg, A.: DTRE – a semi-automatic transformation system, in constructing programs from specifications. In: Moller, B. (ed.) Constructing Programs from Specifications. North Holland (1991)
2. Burstall, R.M., Goguen, J.A.: The semantics of clear, a specification language. In: Proceedings of the 1979 Copenhagen Winter School on Abstract Software Specification, LNCS, vol. 86, pp. 292–332. Springer, Berlin Heidelberg New York (1979)
3. Cohn, P.M.: Free Rings and their Relations. Academic (1985)
4. Ehrig, H.: Bigraphs meet double pushouts. EATCS Bulletin, vol. 78, pp. 72–85 (October 2002)
5. Faber, M., Vogel, P.: The Cohn localization of the free group rings. Math. Proc. Comb. Phil. Soc., III 433 (1992)