eg. A flow chart of a program
def. A graph whose edges are directed is a digraph,
directed graph.
def. A digraph 
when V is the set of vertex
and E is the set of directed edges
on every 
Note
For the purpose of this chapter, we assume that V, the set of
nodes, is nonempty & finite and that there no self loops or
multiple edges in a graph or digraph.
Question