Return whether or not the INDEL is a deletion

def is_deletion(reference_bases, alternate_bases):
    """ Return whether or not the INDEL is a deletion """
    # if multiple alts, it is unclear if we have a transition
    if len(alternate_bases) > 1:
        return False

    if is_indel(reference_bases, alternate_bases):
        # just one alt allele
        alt_allele = alternate_bases[0]
        if alt_allele is None:
            return True
        if len(reference_bases) > len(alt_allele):
            return True
        else:
            return False
    else:
        return False