code: def size(self, t=None):
          
          s = sum(self.degree(t=t).values()) / 2
          return int(s)
docs: 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
          --------
          >>> G = dn.DynGraph()
          >>> G.add_path([0,1,2,3], t=0)
          >>> G.size(t=0)
          3