code: def test_overlay():
      im = mh.demos.load("luispedro", as_grey=1)
      im = mh.stretch(im)
      assert np.all(mh.overlay(im).max(2) == im)
      edges = mh.sobel(im)
  
      im3 = mh.overlay(im, green=edges)
      assert np.all(im3[:, :, 0] == im)
      assert np.all(im3[:, :, 2] == im)
      assert np.all(im3[:, :, 1] >= im)
  
file_path: mahotas/mahotas/tests/test_stretch.py
imports: from mahotas.stretch import stretch
  import mahotas
  import mahotas
  import pytest
  import numpy
label_desc: Use image node_id 9 for calling a global function with example usage: test_overlay() without return types