docs: def _slice(self, level):
          '''Return list of tuples of ident and lineage ident for given level
  (numbered rank)'''
          if level >= len(self.taxonomy):
              raise IndexError('Level greater than size of taxonomy')
          res = []
          for ident in sorted(list(self.keys())):
              res.append((self[ident]['taxref'], self[ident]['lineage'][level]))
          return res