def eval_overlap(n1, n2):
    """
    Return a tuple containing the number of matches (resp.,
    mismatches) between a pair (n1,n2) of overlapping reads
    """
    hang1 = n2["begin"] - n1["begin"]
    overlap = zip(n1["alleles"][hang1:], n2["alleles"])
    match = mismatch = 0
    for (c1, c2) in overlap:
        if c1 == c2:
            match += 1
        else:
            mismatch += 1
    return match, mismatch