docs: def quads_to_tris(quads):
      '''
      Convert quad faces to triangular faces.
  
      quads: An nx4 array.
  
      Return a 2nx3 array.
  
      '''
      import numpy as np
      tris = np.empty((2 * len(quads), 3))
      tris[0::2, :] = quads[:, [0, 1, 2]]
      tris[1::2, :] = quads[:, [0, 2, 3]]
      return tris