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Each vertex can have any hashable object as a label. These are things like
strings, numbers, and tuples. Each edge is given a default label of None, but
if specified, edges can have any label at all. Edges between vertices 
and 
are
represented typically as (u, v, l), where l is the label for the edge.
Note that vertex labels themselves cannot be mutable items:
sage: M = Matrix( [[0,0],[0,0]] )
sage: G = Graph({ 0 : { M : None } })
Traceback (most recent call last):
...
TypeError: mutable matrices are unhashable
However, if one wants to define a dictionary, with the same keys and arbitrary objects for entries, one can make that association:
sage: d = {0 : graphs.DodecahedralGraph(), 1 : graphs.FlowerSnark(), \
2 : graphs.MoebiusKantorGraph(), 3 : graphs.PetersenGraph() }
sage: d[2]
Moebius-Kantor Graph: Graph on 16 vertices
sage: T = graphs.TetrahedralGraph()
sage: T.vertices()
[0, 1, 2, 3]
sage: T.set_vertices(d)
sage: T.get_vertex(1)
Flower Snark: Graph on 20 vertices
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