def get_by_index(self, i):
if i >= self.n:<EOL><INDENT>raise ValueError('<STR_LIT>' % i +<EOL>'<STR_LIT>' % self.n)<EOL><DEDENT>return self._gene_sets[self._gene_set_ids[i]]<EOL>
Look up a gene set by its index.

        Parameters
        ----------
        i: int
            The index of the gene set.

        Returns
        -------
        GeneSet
            The gene set.

        Raises
        ------
        ValueError
            If the given index is out of bounds.