code: def edges(self, sites, permutation=None, input='sites'):
          
          if input == 'sites':
              coords = [site.coords for site in sites]
          elif input == 'coords':
              coords = sites
          # if permutation is None:
          #     coords = [site.coords for site in sites]
          # else:
          #     coords = [sites[ii].coords for ii in permutation]
          if permutation is not None:
              coords = [coords[ii] for ii in permutation]
          return [[coords[ii] for ii in e] for e in self._edges]
docs: Returns the list of edges of this coordination geometry. Each edge is given as a
          list of its end vertices coordinates.