Language: C++

#include <iostream>
#include <iosfwd>
#include <iomanip>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <ctime>
#include <cmath>
#include <cassert>
#include <cctype>
#include <climits>
#include <vector>
#include <bitset>
#include <set>
#include <queue>
#include <stack>
#include <map>
#include <deque>
#include <string>
#include <list>
#include <iterator>
#include <sstream>
#include <complex>
#include <fstream>
#include <functional>
#include <numeric>
#include <utility>
#include <algorithm>
#include <assert.h>
#include <unordered_map>
using namespace std;
// LIVE AND LET OTHERS LIVE.
#define IOS ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(0);
#define endl '\n'
#define stackofi stack<int>
#define queueofi queue<int>  
#define vectoriterator vector<int>::iterator
#define mapiterator map<int,int>::iterator
#define setiterator set<int>:: iterator
#define yes cout<<"YES"<<endl
#define no cout<<"NO"<<endl
// macros for inputting 1-D and 2-D array.
#define stringiterator string:: iterator
#define pb push_back
#define all(c) (c).begin(),(c).end()
#define rall(c) (c).rbegin(),(c).rend()
#define ai(arr,n) for(int i=0;i<n;i++)cin>>arr[i];
#define ao(arr) for(auto wtf:arr) cout<<wtf<<" ";
#define mi(arr,m,n) for(int i=0;i<m;i++){ for(int j=0;j<n;j++) cin>>arr[i][j];}
#define mo(arr,m,n) for(int i=0;i<m;i++){ for(int j=0;j<n;j++) cout<<arr[i][j]<<" "; cout<<endl;}
#define countsetbits(x) builtin_popcount(x)
#define ll long long
#define debugger cout<<"I AM EXECUTING"<<endl
#define testcases int asdf; cin>>asdf; while(asdf--)
#define space cout<<endl
#define vi vector<int>
#define si set<int>
#define vc vector<char>
#define sc set<char>
#define vll vector<long long int>
#define vs vector<string>
#define vpp vector<pair<int,int>> 
#define vppo(prs) for(auto x:prs){cout<<x.first<<" "<<x.second<<endl;}
#define For(,$$,adfdf) for(int  = $$; <adfdf;__++)
#define MOD (ll)1e9+7
ll int mpow(ll int base,ll  int exp) {
  base %= MOD;
  ll int result = 1ll;
  while (exp > 0) {
  if (exp & 1ll) result = ((ll)result * base) % MOD;
  base = ((ll)base * base) % MOD;
  exp >>= 1;
  }
  return result;
}
string sconvert(ll int n)
{
  stringstream ss;
  ss<<n;
  string str = ss.str();
  return str;
}
bool sortbysec(const pair<int,int> &a, 
              const pair<int,int> &b) 
{ 
    return (a.second > b.second); 
}
template<typename KeyType, typename ValueType>  // for finding max element in the map.
std::pair<KeyType,ValueType> get_max( const std::map<KeyType,ValueType>& x ) {
  using pairtype=std::pair<KeyType,ValueType>; 
  return *std::max_element(x.begin(), x.end(), [] (const pairtype & p1, const pairtype & p2) {
        return p1.second < p2.second;
  }); 
}

ll int add(ll int a, ll int b, ll int mod = MOD) {
    if (a + b >= mod) {
        return a + b - mod;
    }
    return a + b;
}
 
ll int sub(ll int a,ll int b,ll int mod = MOD) {
    if (a - b < 0) {
        return a - b + mod;
    }
    return a - b;
}
 
ll int mul(ll int a, ll int b, ll int mod = MOD) {
    return (ll int)(1ll * a * b % mod);
}


void  single()
{
  
  ll int n;
  cin>>n;
  vll arr(n);
  ai(arr,n);
  vll p(n+1);
  p[0]=arr[0];
  For(i,1,n){
    p[i]=p[i-1]+arr[i];
  }
  ll int ans=0;
  For(i,0,n){
    ans = add(ans,mul(arr[i],p[n-1]-p[i]));
  }
  cout<<ans<<endl;
  return;
}
void multiple()
{
  testcases
    {
    single();
  }
}

int main()
{
  IOS;
 // freopen("input.txt","r",stdin);
  //freopen("output.txt","w",stdout);
  single();
}

Compiler output:
a.cc:64:13: error: expected parameter name, found ","
   64 | #define For(,$$,adfdf) for(int  = $$; <adfdf;__++)
      |             ^
a.cc: In function 'void single()':
a.cc:124:7: error: 'i' was not declared in this scope
  124 |   For(i,1,n){
      |       ^
a.cc:124:3: error: 'For' was not declared in this scope
  124 |   For(i,1,n){
      |   ^~~