def _combine_counts(count1, count2):
    """ Sum two counts, but check that if one is 'unknown',
        both are 'unknown'. In those cases, return a single
        value of 'unknown'. """
    if count1 == 'unknown' or count2 == 'unknown':
        assert(count1 == 'unknown' and count2 == 'unknown')
        return 'unknown'
    assert(type(count1) == int and type(count2) == int)
    return count1 + count2