code: def test_deadend():
      """Check if one-node cycles are handled properly. Taken from ticket
      #2906 from @sshraven."""
      G = nx.DiGraph()
  
      G.add_nodes_from(range(5), demand=0)
      G.nodes[4]["demand"] = -13
      G.nodes[3]["demand"] = 13
  
      G.add_edges_from(
          [(0, 2), (0, 3), (2, 1)], capacity=20, weight=0.1
      )
      pytest.raises(nx.NetworkXUnfeasible, nx.network_simplex, G)
  
imports: import bz2
  import importlib.resources
  import os
  import pickle
  import pytest
  import networkx
label_desc: Use image node_id 20 for calling a global function with example usage: test_deadend() without return types