pi_ = g (1, 0, 1, 1, 3, 3)
where
g (q, r, t, k, n, l) =
if 4 * q + r - t < n * t
then n :
g
( 10 * q
, 10 * (r - n * t)
, t
, k
, div (10 * (3 * q + r)) t - 10 * n
, l)
else g
( q * k
, (2 * q + r) * l
, t * l
, k + 1
, div (q * (7 * k + 2) + r * l) (t * l)
, l + 2)