code: def test_merge():
      a = pd.Series(
          index=pd.date_range("2010-01-01", periods=5),
          data=100,
          name="a",
      )
      b = pd.Series(
          index=pd.date_range("2010-01-02", periods=5),
          data=200,
          name="b",
      )
      actual = ffn.merge(a, b)
  
      assert "a" in actual
      assert "b" in actual
      assert len(actual) == 6
      assert len(actual.columns) == 2
      assert np.isnan(actual["a"][-1])
      assert np.isnan(actual["b"][0])
      assert actual["a"][0] == 100
      assert actual["a"][1] == 100
      assert actual["b"][-1] == 200
      assert actual["b"][1] == 200
  
      old = actual
      old.columns = ["c", "d"]
  
      actual = ffn.merge(old, a, b)
  
      assert "a" in actual
      assert "b" in actual
      assert "c" in actual
      assert "d" in actual
      assert len(actual) == 6
      assert len(actual.columns) == 4
      assert np.isnan(actual["a"][-1])
      assert np.isnan(actual["b"][0])
      assert actual["a"][0] == 100
      assert actual["a"][1] == 100
      assert actual["b"][-1] == 200
      assert actual["b"][1] == 200
  
imports: import ffn
  import pandas
  import numpy
  from pytest import fixture
  from numpy.testing import assert_almost_equal
label_desc: Use image node_id 17 for calling a global function with example usage: test_merge() without return types