code: def has_node(self, n, t=None):
          
          if t is None:
              try:
                  return n in self._node
              except TypeError:
                  return False
          else:
              deg = list(self.degree([n], t).values())
              if len(deg) > 0:
                  return deg[0] > 0
              else:
                  return False
docs: Return True if the graph, at time t, contains the node n.
  
          Parameters
          ----------
          n : node
          t : snapshot id (default None)
                  If None return the presence of the node in the flattened graph.
  
          Examples
          --------
          >>> G = dn.DynGraph()   # or DiGraph, MultiGraph, MultiDiGraph, etc
          >>> G.add_path([0,1,2], t=0)
          >>> G.has_node(0, t=0)
          True
  
          It is more readable and simpler to use
  
          >>> 0 in G
          True