code: def strip_unknown_features(self):
          
          if not self.db:
              return self
          ind = []
          for i, gene_id in enumerate(self.data.index):
              try:
                  self.db[gene_id]
                  ind.append(i)
              except gffutils.FeatureNotFoundError:
                  pass
          ind = np.array(ind)
          return self.__class__(self.data.ix[ind], **self._kwargs)
docs: Remove features not found in the `gffutils.FeatureDB`.  This will
          typically include 'ambiguous', 'no_feature', etc, but can also be
          useful if the database was created from a different one than was used
          to create the table.