def strip_unknown_features(self):
        """
        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.
        """
        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)
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.