In category theory, if C and D are both categories, then a functor from C to D maps every morphism in C to some morphism in D. You can think of a functor as a set of "morphism-to-morphism"-morphisms. Furthermore, if the functor maps from category C to category C (i.e., D=C), this is called an endofunctor.
In mm-ADT, in the type subgraph of the obj graph, the vertices denote types and a path (of edges) back to the root of the graph (a ctype root) denotes a type definition. These edges are labeled with instructions from inst, where a path is a sequence of operations that morph one type into another type (see The Type). In abstract algebra, the type subgraph is called a (generalized) Cayley graph as the obj graph captures the structure of the underlying inst monoid. N