Prepare a class model to describe directed graphs. A directed graph consists of a set of vertices and a set of

edges. Edges connect pairs of vertices. All edges

are oriented. Your model should capture only the

structure of graphs $($i$.$e$.$ connectivity$)$ and need

not be concerned with layout such as location of

vertices or lengths of edges. Figure $1\mathrm{.}1$ shows a

typical directed graph.