code: def test_main_shell(self):
      main_shell_subgraph = nx.k_shell(self.H)
      assert sorted(main_shell_subgraph.nodes()) == [2, 4, 5, 6]
  
imports: import pytest
  import networkx
  from networkx.utils import nodes_equal
label_desc: Use image node_id 14 for calling the TestCore obj's underlying member method code with example usage: obj.test_main_shell() without return types