def s_metric(G):
"""Return the s-metric of graph.
The s-metric is defined as the sum of the products deg(u)*deg(v)
for every edge (u,v) in G.
Parameters
----------
G : graph
The graph used to compute the s-metric.
Returns
-------
s : float
The s-metric of the graph.
References
----------
.. [1] Lun Li, David Alderson, John C. Doyle, and Walter Willinger,
Towards a Theory of Scale-Free Graphs:
Definition, Properties, and Implications (Extended Version), 2005.
http://arxiv.org/abs/cond-mat/0501169
"""
return sum([G.degree(u)*G.degree(v) for (u,v) in G.edges_iter()])
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