code: def facets_area(self):
          
          # avoid thrashing the cache inside a loop
          area_faces = self.area_faces
          # sum the area of each group of faces represented by facets
          # use native python sum in tight loop as opposed to array.sum()
          # as in this case the lower function call overhead of
          # native sum provides roughly a 50% speedup
          areas = np.array([sum(area_faces[i])
                            for i in self.facets],
                           dtype=np.float64)
          return areas
docs: Return an array containing the area of each facet.
  
          Returns
          ---------
          area : (len(self.facets),) float
            Total area of each facet (group of faces)