docs: def index_genome_alignment_by_locus(fh, out_fh, verbose=False):
    """Build an index for a genome alig. using coords in ref genome as keys."""
    bound_iter = functools.partial(genome_alignment_iterator,
                                   reference_species="hg19", index_friendly=True)
    hash_func = JustInTimeGenomeAlignmentBlock.build_hash
    idx = IndexedFile(fh, bound_iter, hash_func)
    idx.write_index(out_fh, verbose=verbose)