Language: C++

#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <array>
#include <vector>
#include <deque>
#include <list>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <stack>
#include <queue>
#include <bitset>
#include <tuple>
#include <cmath>
#include <complex>
#include <algorithm>
#include <utility>
#include <regex>
#include <cstdint>
#include <numeric>
#include <functional>
#include <cassert>

using namespace std;
using ll = long long;
using ull = unsigned long long;

constexpr ll INF = 1'000'000'000'000'000'000;
namespace utils{
#define ALL(x) begin(x), end(x)
#define RALL(x) rbegin(x), rend(x)
    template<class T, class Compare> using p_queue = priority_queue<T, vector<T>, Compare>;
    template<class T> using min_queue = p_queue<T, greater<T>>;
    template<class T> using max_queue = p_queue<T, less<T>>;

    template<class T> bool up_min(T& X, const T& A){ if(X > A) {X = A; return true;} return false; }
    template<class T> bool up_max(T& X, const T& A){ if(X < A) {X = A; return true;} return false; }

    ll operator"" _64(unsigned long long x){ return ll(x); }
    ull operator"" _64u(unsigned long long x){ return ull(x); }

    template<class T>
    vector<T> make_mdvector(int n, T t){
        return vector<T>(n, t);
    }
    template<class ...Ts>
    auto make_mdvector(int n, Ts ... ts){
        return vector<decltype(make_mdvector(ts...))>(n, make_mdvector(ts...));
    }

    inline ll bit(int x){
        return 1ll << x;
    }

    template<class T>
    istream& operator>>(istream& is, vector<T>& v){
        for (auto &&x : v) { is >> x; } return is;
    }

    template<class T>
    ostream& operator<<(ostream& os, const vector<T>& v){
        for (auto &&x : v) { os << x << ' '; } cout << endl; return os ;
    }
}
using namespace utils;

class solver{
    istream& is;
    ostream& os;

public:
    solver(istream& I, ostream& O) :is(I), os(O) {}
    ll N;
    void run();
};

int main(int argc, char *argv[]) {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << setprecision(16) << scientific;
#ifdef XELMH
    string test_cases = "test_D.txt";
    cerr << test_cases << " -->" << endl;
    auto fs = fstream(test_cases, fstream::in);
    int loop = 0;
    while(fs) {
        loop++;
        cout << '#' << loop << "#------\n";
        solver(fs, cout).run();
    }
    if(loop <= 1) {
        cout << "===" << endl;
        while(cin) solver(cin, cout).run();
    }
#else
    solver(cin, cout).run();
#endif
    return 0;
}

namespace xelmh_RCR {
    class RCR {
        // Residue Class Ring
        static constexpr ll NIL = -1;
        static constexpr int64_t MOD = 1'000'000'007;
//        static constexpr int64_t MOD = 998'244'353;

        ll val;
    public:
        RCR(ll x) : val(x >= 0 ? x % MOD : MOD - (-x) % MOD) {}
        RCR(signed x) : val(x >= 0 ? x % MOD : MOD - (-x) % MOD) {}
        RCR(ull x) : val(x % MOD) {}
        RCR(unsigned x) : val(x % MOD) {}
        RCR() : val{NIL} {}

        bool is_nil() { return val == NIL; }

        RCR pow(ll n) const {
            ll r = 1, a = val;
            while (n) {
                if (n & 1) {
                    r *= a;
                    r %= MOD;
                }
                a *= a;
                a %= MOD;
                n /= 2;
            }
            return {r};
        }
        RCR inv() const { /* return Inverse */ assert(val != 0);
            return this->pow(MOD - 2);
        }

        void operator+=(const RCR &a) {
            val += a.val;
            if (val >= MOD) val -= MOD;
        }
        void operator-=(const RCR &a) {
            val -= a.val;
            if (val < 0) val += MOD;
        }
        void operator*=(const RCR &a) {
            val *= a.val;
            val %= MOD;
        }
        void operator/=(const RCR &a) {
            val *= a.inv().val;
            val %= MOD;
        }

        RCR operator+(const RCR &x) const {
            ll v = val + x.val;
            return {v};
        }
        RCR operator-(const RCR &x) const {
            ll v = val - x.val;
            return {v};
        }
        RCR operator*(const RCR &x) const {
            ll v = val * x.val;
            return {v};
        }
        RCR operator/(const RCR &x) const {
            ll v = val * x.inv().val;
            return {v};
        }

        bool operator==(const RCR &x) const { return x.val == val; }
        bool operator!=(const RCR &x) const { return x.val != val; }

        friend istream &operator>>(istream &is, RCR &r) {
            ll tmp;
            is >> tmp;
            r = tmp;
            return is;
        }
        friend ostream &operator<<(ostream &os, const RCR &r) { return os << r.val; }
    };

    RCR fact(ll N) {
        /* N -> N! */
        RCR ans = 1;
        while(N > 0) ans *= N--;
        return ans;
    }

    /* N, k -> N C k */
    RCR choose(ll N, ll k) {
        // N! / k!(N-k!)
        RCR ans = fact(N) / (fact(k) * fact(N - k));
        return ans;
    }

    RCR operator "" _mod(unsigned long long x) {
        return RCR{x};
    }
}
using namespace xelmh_RCR;


void solver::run(){
    ll X, Y;
    is >>X >> Y;
    if(!is) return;
    ll k = X+Y;
    if(k % 3 != 0){
        os << "0\n";
        return;
    }
    k /= 3;
    X-=k; Y-=k;
    if(X < 0 or Y < 0){
        os << "0\n";
        return;
    }
    RCR ans = choose(X+Y, X);
    os << ans << endl;
}
g

Compiler output:
a.cc:223:1: error: 'g' does not name a type
  223 | g
      | ^