func next_term(k int64) {
y2 := k*2 + 1
tmp1 = numer.Shl(1)
bignum.Iadd(accum, accum, tmp1)
bignum.Iscale(accum, y2)
bignum.Iscale(numer, k)
bignum.Iscale(denom, y2)
}
func eliminate_digit(d int64) {
bignum.Isub(accum, accum, denom.Mul1(d))
bignum.Iscale(accum, 10)
bignum.Iscale(numer, 10)
}