code: def flat_mesh(x_s, y_s, z=0):
      """Create a flat horizontal mesh."""
      x, y = np.mgrid[:x_s, :y_s]
      x, y = x.ravel(), y.ravel()
      z = np.ones(len(x)) * z
      vertices = np.asarray([x, y, z]).T
      triangulation = Delaunay(vertices[:, :2]).simplices
      mesh = [vertices, triangulation]
      return mesh
  
imports: import numpy
  from scipy.spatial import Delaunay
  from nilearn.conftest import _rng
label_desc: Use image node_id 2 for calling a global function with example usage: flat_mesh(x_s, y_s, z) and returns: mesh