code: def test_tricontourf_decreasing_levels():
      # github issue 5477.
      x = [0.0, 1.0, 1.0]
      y = [0.0, 0.0, 1.0]
      z = [0.2, 0.4, 0.6]
      plt.figure()
      with pytest.raises(ValueError):
          plt.tricontourf(x, y, z, [1.0, 0.0])
  
imports: import numpy
  from numpy.testing import assert_array_equal, assert_array_almost_equal, assert_array_less
  import numpy.ma.testutils
  import pytest
  import matplotlib
  import matplotlib.pyplot
  import matplotlib.tri
  from matplotlib.path import Path
  from matplotlib.testing.decorators import image_comparison, check_figures_equal
label_desc: Use image node_id 31 for calling a global function with example usage: test_tricontourf_decreasing_levels() without return types