code: def test_ndlayout_overlay_element(self):
      items = [
          (0, self.view1),
          (1, self.view2),
          (2, self.view3),
          (3, self.view2),
      ]
      grid = NdLayout(items)
      hline = HLine(0)
      overlaid_grid = grid * hline
      expected = NdLayout([(k, v * hline) for k, v in items])
      self.assertEqual(overlaid_grid, expected)
  
imports: import numpy
  import pytest
  from holoviews import AdjointLayout, Element, GridSpace, HoloMap, Layout, NdLayout, Overlay
  from holoviews.element import Curve, HLine, Image
  from holoviews.element.comparison import ComparisonTestCase
label_desc: Use image node_id 2 for calling the NdLayoutTest obj's underlying member method code with example usage: obj.test_ndlayout_overlay_element() without return types