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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def size(self, t=None):
 """Return the number of edges at time t. Parameters t : snapshot id (default=None) If None will be returned the size of the flattened graph. Returns ------- nedges : int The number of edges See Also -------- number_of_edges Examples -------- 3 """

        s = sum(self.degree(t=t).values()) / 2
        return int(s)