import random
from copy import deepcopy


def rls(n):
    if n <= 0:
        return []
    else:
        symbols = list(range(1, n+1))
        square = _rls(symbols)
        return _shuffle_transpose_shuffle(square)


def _shuffle_transpose_shuffle(matrix):
    square = deepcopy(matrix)
    random.shuffle(square)
    trans = list(zip(*square))
    random.shuffle(trans)
    return trans


def _rls(symbols):
    n = len(symbols)
    if n == 1:
        return [symbols]
    else:
        sym = random.choice(symbols)
        symbols.remove(sym)
        square = _rls(symbols)
        square.append(square[0].copy())
        for i in range(n):
            square[i].insert(i, sym)
        return square


def _to_text(square):
    if square:
        width = max(len(str(sym)) for row in square for sym in row)
        txt = '\n'.join(' '.join("{}".format(sym).rjust(width, " ") for sym in row)
                        for row in square)
    else:
        txt = ''
    return txt


def _check(square):
    transpose = list(zip(*square))
    assert _check_rows(square) and _check_rows(transpose), \
        "Not a Latin square"


def _check_rows(square):
    if not square:
        return True
    set_row0 = set(square[0])
    return all(len(row) == len(set(row)) and set(row) == set_row0
               for row in square)


if __name__ == '__main__':
    T = int(input().strip())

    resultArr = []

    for x in range(1, T+1):
        n, k = list(map(int, input().strip().split()))

        arrayIDX = [n for idx in range(50)]

        ltsArr = []

        for i in arrayIDX:
            square = rls(i)
            ltsArr.append(square)
            _check(square)

        # if possible to have lts of k
        for lts in ltsArr:
            temp = 0

            for rIDX in range(len(lts)):
                temp += lts[rIDX][rIDX]

            if temp == k:
                result = "POSSIBLE"
                break
            else:
                result = "IMPOSSIBLE"

        if result == "POSSIBLE":
            resultArr.append([x, result, _to_text(lts)])
        else:
            resultArr.append([x, result])

        print("Case #{}: {}".format(x, result))

        if result == "POSSIBLE":
            print(_to_text(lts))