def _nth_str(n):
    """Return posessive form of numeral `n`: 1st, 2nd, 3rd, etc."""
    if n % 10 == 1 and n % 100 != 11:
        return "%dst" % n
    if n % 10 == 2 and n % 100 != 12:
        return "%dnd" % n
    if n % 10 == 3 and n % 100 != 13:
        return "%drd" % n
    return "%dth" % n