H. Ehrig, and H. Kreowski. Category Theory Applied to Computation and Control, volume 25 of Lecture Notes in Computer Science, page 144-150. Springer, (1974)
H. Ehrig, W. Kühnel, and M. Pfender. Category Theory Applied to Computation and Control, volume 25 of Lecture Notes in Computer Science, page 137-143. Springer, (1974)