#include <iostream>
#include <vector>
using namespace std;
 
bool checkRectangles(vector<string>& grid, int n, int m) {
    for (int i = 0; i < n - 1; ++i) {
        for (int j = 0; j < m - 1; ++j) {
            if (grid[i][j] != grid[i + 1][j] && grid[i][j] != grid[i][j + 1]) {
                return false;
            }
        }
    }
    return true;
}
 
int main() {
    int t;
    cin >> t;
 
    while (t--) {
        int n, m;
        cin >> n >> m;
        vector<string> grid(n);
        for (int i = 0; i < n; ++i) {
            cin >> grid[i];
        }
 
        bool possible = checkRectangles(grid, n, m);
 
        if (possible) {
            cout << "YES" << endl;
        } else {
            cout << "NO" << endl;
        }
    }
 
    return 0;
}
 

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