def count_overlaps(overlap_tuples):
    """
    Given a list of overlap tuples as output by `compute_interval_overlaps()`,
    returns a tuple of the counts of the number of a intervals with overlaps 
    and b intervals with overlaps.
    """
    a_set = set([])
    b_set = set([])
    for tup in overlap_tuples:
        a_set.add(tup[0])
        b_set.add(tup[1])
    return len(a_set), len(b_set)
