J. Jahnke, W. Schäfer, and A. Zündorf. Proc. European Software Engineering Conf. (ESEC), volume 1013 of Lecture Notes in Computer Science, page 193--210. Springer-Verlag, (1997)
J. Goguen. Proc. Conf. Discrete Mathematics and Theoretical Computer Science, volume 21 of Australian Computer Science Communications, page 35--59. Springer-Verlag, (1999)