docs: def _build_index(maf_strm, ref_spec):
    """Build an index for a MAF genome alig file and return StringIO of it."""
    idx_strm = StringIO.StringIO()
    bound_iter = functools.partial(genome_alignment_iterator,
                                   reference_species=ref_spec)
    hash_func = JustInTimeGenomeAlignmentBlock.build_hash
    idx = IndexedFile(maf_strm, bound_iter, hash_func)
    idx.write_index(idx_strm)
    idx_strm.seek(0)  # seek to the start
    return idx_strm