def one2two(k):
    """Inverts the two2one map --- given a natural number, return its
    corresponding (x,y) pair."""
    diag = int((sqrt(1 + 8*k) - 1) / 2)
    offset = k - diag * (diag + 1) / 2
    return (diag - offset, offset)
Inverts the two2one map --- given a natural number, return its
    corresponding (x,y) pair.
recipes/Python/52271_How_take_cross_product_two_infinite/recipe-52271.py