docs: def create_horizontal_plane():
      '''
      Creates a horizontal plane.
      '''
      v = np.array([
          [1., 0., 0.],
          [-1., 0., 0.],
          [0., 0., 1.],
          [0., 0., -1.]
      ])
      f = [[0, 1, 2], [3, 1, 0]]
      return Mesh(v=v, f=f)