code: def test_flat_mesh(xy):
      points, triangles = _flat_mesh(xy[0], xy[1])
      a, b, c = points[triangles[0]]
      n = np.cross(b - a, c - a)
      assert np.allclose(n, [0.0, 0.0, 1.0])
  
imports: import os
  import tempfile
  import warnings
  import nibabel
  import numpy
  import pytest
  from nibabel import gifti
  from numpy.testing import assert_array_almost_equal, assert_array_equal
  from scipy.spatial import Delaunay
  from scipy.stats import pearsonr
  from nilearn import datasets, image
  from nilearn._utils import data_gen
  from nilearn.image import resampling
  from nilearn.surface import Mesh, Surface, load_surf_data, load_surf_mesh, surface
  from nilearn.surface.surface import _gifti_img_to_mesh, _load_surf_files_gifti_gzip
  from nilearn.surface.tests._testing import flat_mesh, generate_surf, z_const_img
label_desc: Use image node_id 18 for calling a global function with example usage: test_flat_mesh(xy) without return types