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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 execution_order(self, refcounts):
 """ Return a topologically-sorted iterator over the terms in ``self`` which need to be computed. """

        return iter(nx.topological_sort(
            self.graph.subgraph(
                {term for term, refcount in refcounts.items() if refcount > 0},
            ),
        ))