Method gcd

Miscellaneous/BigInt.cpp:457–459

    return a.empty() || (a.size() == 1 && !a[0]);
  }
  friend BigInt gcd(const BigInt &a, const BigInt &b) {
    return b.zero() ? a : gcd(b, a % b);
  }
};
BigInt power(BigInt a, ll k) {
  BigInt ans = 1;