accum = big.NewInt(0)
	denom = big.NewInt(1)
	ten   = big.NewInt(10)
)

func extractDigit() int64 {
	if big.CmpInt(numer, accum) > 0 {
		return -1
	}
	tmp1.Lsh(numer, 1).Add(tmp1, numer).Add(tmp1, accum)
	big.DivModInt(tmp1, tmp2, tmp1, denom)
	tmp2.Add(tmp2, numer)
	if big.CmpInt(tmp2, denom) >= 0 {
		return -1
	}
	return tmp1.Int64()
}

func nextTerm(k int64) {
	y2 := k*2 + 1