def group_reads_by_allele(allele_reads):
    """
    Returns dictionary mapping each allele's nucleotide sequence to a list of
    supporting AlleleRead objects.
    """
    allele_to_reads_dict = defaultdict(list)
    for allele_read in allele_reads:
        allele_to_reads_dict[allele_read.allele].append(allele_read)
    return allele_to_reads_dict
Returns dictionary mapping each allele's nucleotide sequence to a list of
    supporting AlleleRead objects.