 
Summary: Graph concepts
Graphs are made up by vertices (nodes) and edges (links).
An edge connects two vertices, or a vertex with itself loop.
AC, AC  multiple edges
BB loop
The shape of the graph does not
matter, only the way the nodes are
connected to each other.
Simple graph  does not have loops and multiple edges.
Further reading:
http://www.utm.edu/departments/math/graph/glossary.html
Chapters IIII (p 119) of "The structure and function of complex
networks"
Symmetrical and directed graphs
Two distinct types of edges: symmetrical and directed (also called arcs).
Two different graph frameworks: graph, digraph.
In the digraph framework a symmetrical edge means the superposition
of two opposite directed edges.
Node degrees
Node degree: the number of edges connected to the
