Class  Description 

Graph<V,E> 
Graph, where all edges and vertices are stored inmemory.
Internally, this is a directed graph with adjacency list representation; however, if undirected edges are added, these edges are duplicated internally to allow for fast lookup. Depending on the value of allowMultipleEdges , this graph implementation may or may not allow
multiple edges between any two adjacent nodes. 
VertexSequence<V> 
A vertex sequence represents a sequences of vertices in a graph
