code: def test_core_number_multigraph(self):
      G = nx.complete_graph(3)
      G = nx.MultiGraph(G)
      G.add_edge(1, 2)
      with pytest.raises(
          nx.NetworkXNotImplemented,
          match="not implemented for multigraph type",
      ):
          nx.core_number(G)
  
imports: import pytest
  import networkx
  from networkx.utils import nodes_equal
label_desc: Use image node_id 5 for calling the TestCore obj's underlying member method code with example usage: obj.test_core_number_multigraph() without return types