#include <bits/stdc++.h>

using namespace std;

void initMat(int** mat, int n) {
    int beg = 0;
    for (int i = 0; i < n; i ++) {
        for (int j = 0; j < n; j ++) {
            mat[i][j] = ((beg + j) % n) + 1;
        }
        beg ++;
    }
}

void printMat(int** mat, int n) {
    for (int i = 0; i < n; i ++) {
        cout << endl;
        for (int j = 0; j < n; j ++) {
            cout << mat[i][j];
            if (j != n - 1)
                cout << " ";
        }
    }
}

bool initDiag(int* diag, int n, int k) {

    for (int i = 0; i < n; i ++){
        diag[i] ++;
        k --;
    }

    for (int i = n - 1; i >= 0 && k > 0; i --) {
        while (diag[i] < n && k > 0){
            diag[i] ++;
            k --;
        }
    }


    bool same = true;
    int tmpVal = diag[0];
    for (int i = 1; i < n - 1; i ++)
        if (tmpVal != diag[i]) {
            same = false;
            break;
        }
    if (same && diag[n-1] != tmpVal) {
        diag[n - 2] ++;
        diag[n - 1] --;
    }
    same = true;
    tmpVal = diag[1];
    for (int i = 1; i < n; i ++){
        if (tmpVal != diag[i]){
            same = false;
            break;
        }
    }
    if (same && diag[0] != tmpVal) {
        diag[0] ++;
        diag[1] --;
    }

}

void solve(int** mat, int* diag, int n) {
    for (int i = 0; i < n; i ++) {
        int target = diag[i];
        int targetRow = 0;
        for (int j = 0; j < n; j ++) {
            if (mat[j][i] == target){
                targetRow = j;
                break;
            }
        }

        for (int j = 0; j < n; j ++) {
            int temp = mat[i][j];
            mat[i][j] = mat[targetRow][j];
            mat[targetRow][j] = temp;
        }

    }
}

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);

    int t;
    cin >> t;

    for (int i = 0; i < t; i ++) {
        int n, k;
        cin >> n >> k;
        if (k < n || k > n*n || k == n + 1 || k == n * n - 1){
            cout << "Case #" << i + 1 << ": IMPOSSIBLE";
            if (i != t - 1)
                cout << endl;
            continue;
        }
        int** mat;
        mat = new int*[n];
        for (int i = 0; i < n; i ++)
            mat[i] = new int[n];
        for (int i = 0; i < n; i ++)
            for (int j = 0; j < n; j ++)
                mat[i][j] = 0;
        int diag[n] = {0};
        initMat(mat, n);
        initDiag(diag, n, k);
        solve(mat, diag, n);

        cout << "Case #" << i + 1 << ": POSSIBLE";
        printMat(mat, n);
        if (i != t - 1)
            cout << endl;
    }
}