code: def test_subtree_union():
      # See https://github.com/networkx/networkx/pull/3224
      # (35db1b551ee65780794a357794f521d8768d5049).
      # Test if subtree unions hare handled correctly by to_sets().
      uf = nx.utils.UnionFind()
      uf.union(1, 2)
      uf.union(3, 4)
      uf.union(4, 5)
      uf.union(1, 5)
      assert list(uf.to_sets()) == [{1, 2, 3, 4, 5}]
  
imports: import networkx
label_desc: Use image node_id 2 for calling a global function with example usage: test_subtree_union() without return types