docs: def save_all(polyfile):
          """Save all polygon filters"""
          nump = len(PolygonFilter.instances)
          if nump == 0:
              raise PolygonFilterError("There are not polygon filters to save.")
          for p in PolygonFilter.instances:
              # we return the ret_obj, so we don't need to open and
              # close the file multiple times.
              polyobj = p.save(polyfile, ret_fobj=True)
          polyobj.close()