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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 compute_dominance_frontier(graph, domtree):
 """ Compute a dominance frontier based on the given post-dominator tree. This implementation is based on figure 2 of paper An Efficient Method of Computing Static Single Assignment Form by Ron Cytron, etc. :param graph: The graph where we want to compute the dominance frontier. :param domtree: The dominator tree :returns: A dict of dominance frontier """


    df = {}

    # Perform a post-order search on the dominator tree
    for x in networkx.dfs_postorder_nodes(domtree):

        if x not in graph:
            # Skip nodes that are not in the graph
            continue

        df[x] = set()

        # local set
        for y in graph.successors(x):
            if x not in domtree.predecessors(y):
                df[x].add(y)

        # up set
        if x is None:
            continue

        for z in domtree.successors(x):
            if z is x:
                continue
            if z not in df:
                continue
            for y in df[z]:
                if x not in list(domtree.predecessors(y)):
                    df[x].add(y)

    return df