ABSTRACT
A category is said to have semi-pullbacks if, whenever f : a → b, g : c → b is a pair of morphisms with the same target, there exists in the category an object d together with morphisms r : d → a, s : d → c such that f ◦ r = g◦s. The existence of semi-pullbacks makes sure that bisimulations, which are defined as spans of morphisms, are transitive; bisimulations will be introduced in Chapter 5.
