Peter J. Freyd Peter W. O'Hearn A. John Power Makoto Takeyama R. Street Robert D. Tennent 49-76 1999 228 Theor. Comput. Sci. 1-2 db/journals/tcs/tcs228.html#Freyd99a http://dx.doi.org/10.1016/S0304-3975(98)00354-5