code: def subsets(self, nid, contract=True):
          
          n = self.node(nid)
          subsets = []
          meta = self._meta(nid)
          if 'subsets' in meta:
              subsets = meta['subsets']
          else:
              subsets = []
          if contract:
              subsets = [self._contract_subset(s) for s in subsets]
          return subsets
docs: Retrieves subset ids for a class or ontology object