code: def test_first_label():
      """Test the functionality of the first_label argument."""
      T1 = nx.path_graph(3)
      T2 = nx.path_graph(2)
      actual = nx.join_trees([(T1, 0), (T2, 0)], first_label=10)
      expected_nodes = set(range(10, 16))
      assert set(actual.nodes()) == expected_nodes
      assert set(actual.neighbors(10)) == {11, 14}
  
file_path: networkx/networkx/algorithms/tree/tests/test_operations.py
imports: from itertools import chain
  import networkx
  from networkx.utils import edges_equal, nodes_equal
label_desc: Use image node_id 5 for calling a global function with example usage: test_first_label() without return types