def non_interactions(graph, t=None):
    """Returns the non-existent edges in the graph at time t.

        Parameters
        ----------

        graph : NetworkX graph.
            Graph to find non-existent edges.

        t : snapshot id (default=None)
            If None the non-existent edges are identified on the flattened graph.


        Returns
        -------
        non_edges : iterator
            Iterator of edges that are not in the graph.
        """
    nodes = set(graph)
    while nodes:
        u = nodes.pop()
        for v in (nodes - set(graph[u])):
            yield u, v
