Some Fundamental Algebraic Tools for the Semantics of Computation. Part 1: Comma Categories, Colimits, Signatures and Theories.

Joseph A. Goguen Rod M. Burstall Some Fundamental Algebraic Tools for the Semantics of Computation. Part 1: Comma Categories, Colimits, Signatures and Theories. 175-209 1984 31 Theor. Comput. Sci. db/journals/tcs/tcs31.html#GoguenB84