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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 neighbors_iter(self, n, t=None):
 """Return an iterator over all neighbors of node n at time t. Parameters n : node A node in the graph t : snapshot id (default=None) If None will be returned an iterator over the neighbors of the node on the flattened graph. Examples -------- [1] """

        try:
            if t is None:
                return iter(self._adj[n])
            else:
                return iter([i for i in self._adj[n] if self.__presence_test(n, i, t)])
        except KeyError:
            raise nx.NetworkXError("The node %s is not in the graph." % (n,))