#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  int max1 = 0, max2 = 0;
  for (int i = 0; i < n; i++) {
    char c;
    cin >> c;
    if (c == '+') {
      int x, y;
      cin >> x >> y;
      int value1 = (x > y) ? x : y;
      int value2 = (x < y) ? x : y;
      if (value2 > max2) max2 = value2;
      if (value1 > max1) max1 = value1;
    } else {
      int x, y;
      cin >> x >> y;
      int value1 = (x > y) ? x : y;
      int value2 = (x < y) ? x : y;
      if (max2 <= value2 && max1 <= value1)
        cout << "YES" << endl;
      else
        cout << "NO" << endl;
    }
  }
  cerr << "Time : " << (double)clock() / (double)CLOCKS_PER_SEC << "s\n";
  return 0;
}
