code: def neighbour_and_arc_simplices(self):
          
          nt, ltri, lct, ierr = _tripack.trlist(self.lst, self.lptr, self.lend, nrow=9)
          if ierr != 0:
              raise ValueError('ierr={} in trlist\n{}'.format(ierr, _ier_codes[ierr]))
          ltri = ltri.T[:nt] - 1
          return ltri[:,3:6], ltri[:,6:]
docs: Get indices of neighbour simplices for each simplex and arc indices.
          Identical to get_neighbour_simplices() but also returns an array
          of indices that reside on boundary hull, -1 denotes no neighbour.