code: def test_is_not_forest(self):
      assert not nx.is_forest(self.N4)
      assert not nx.is_forest(self.N6)
      assert not nx.is_forest(self.NF1)
  
imports: import pytest
  import networkx
label_desc: Use image node_id 9 for calling the TestTreeRecognition obj's underlying member method code with example usage: obj.test_is_not_forest() without return types