code: def test_moment_conversion(mom):
      # this was initially written for an old version of moment_helpers
      # I'm not sure whether there are not redundant cases after moving functions
  
      # test moment -> cumulant
      assert_equal(mnc2cum(mc2mnc(mom[0])), mom[1])
      assert_equal(mnc2cum(mom[0]), mom[2])
      if len(mom) <= 4:
          assert_equal(mc2cum(mom[0]), mom[1])
  
      # test   cumulant -> moment
      assert_equal(cum2mc(mom[1]), mom[0])
      assert_equal(mc2mnc(cum2mc(mom[2])), mom[0])
      if len(mom) <= 4:
          assert_equal(cum2mc(mom[1]), mom[0])
  
      # round trip: mnc -> cum -> mc == mnc -> mc,
      assert_equal(cum2mc(mnc2cum(mom[0])), mnc2mc(mom[0]))
  
      # round trip: mc -> mnc -> mc ==  mc,
      assert_equal(mc2mnc(mnc2mc(mom[0])), mom[0])
  
      if len(mom[0]) == 4:
          # round trip: mc -> mvsk -> mc ==  mc
          assert_equal(mvsk2mc(mc2mvsk(mom[0])), mom[0])
  
file_path: statsmodels/statsmodels/stats/tests/test_moment_helpers.py
imports: import numpy
  import pytest
  from numpy.testing import assert_almost_equal, assert_, assert_equal
  from statsmodels.stats import moment_helpers
  from statsmodels.stats.moment_helpers import cov2corr, mvsk2mc, mc2mvsk, mnc2mc, mc2mnc, cum2mc, mc2cum, mnc2cum
label_desc: Use image node_id 2 for calling a global function with example usage: test_moment_conversion(mom) without return types