The minimum vertex (DFS-number) in a frond contained in Ri.

def fn_x(i, dfs_data):
    """The minimum vertex (DFS-number) in a frond contained in Ri."""
    try:
        return R(i, dfs_data)['x']
    except Exception as e:
        # Page 17 states that if Ri is empty, then we take xi to be n
        return dfs_data['graph'].num_nodes()