import sys
import itertools
from typing import List


def get_input_form_stdin():
    contents = []
    while True:
        try:
            line = input()
        except EOFError:
            break
        contents.append(line)
    return contents


def get_input_from_file(input_file_name):
    input_file = open(input_file_name)
    contents = input_file.readlines()
    return contents


def compare_char(a, b):
    if ord(a) == ord(b):
        return 0
    elif ord(a) < ord(b):
        return 1
    return -1


def least_char(input_string):
    min_value = input_string[0]
    min_pos = 1
    for index, ch in enumerate(input_string, 1):
        if ch < min_value:
            min_pos = index
            min_value = ch
    return min_pos, min_value


def is_sorted(sub_string_list):
    sorted_list = sub_string_list.copy()
    sorted_list.sort()
    return sorted_list == sub_string_list


def increase_value(to_be_mod, increase_compare, index):
    letters_bigger = (x for x in to_be_mod[index:] if ord(x) >= ord(increase_compare[index]))
    letters_bigger_array = []
    for x in letters_bigger:
        letters_bigger_array.append((x, to_be_mod.index(x)))
    if len(letters_bigger_array) == 0:
        return False
    candidate_index = letters_bigger_array[0][1]
    return to_be_mod[:index] + to_be_mod[candidate_index] + to_be_mod[index:candidate_index] + to_be_mod[
                                                                                               candidate_index + 1:]


def resolve_case(index, num_substrings: int, sub_string_list):
    if is_sorted(sub_string_list):
        return True, sub_string_list

    is_possible = False
    solution = sub_string_list
    solution[0] = ''.join(sorted(solution[0]))
    index = 1
    while index < len(solution):
        a = solution[index - 1]
        solution[index] = ''.join(sorted(solution[index]))

        if is_sorted([a, solution[index]]):
            index += 1
            continue
        char_index = 0
        while char_index + 1 <= len(solution[index]) and not is_sorted([a, solution[index]]):
            inc = increase_value(solution[index], a, char_index)
            if inc == False:
                return False, []
            solution[index] = inc
            char_index += 1
        index += 1
    is_possible = is_sorted(solution)
    return is_possible, solution


if __name__ == '__main__':

    with_files = False
    input_file_name = ""
    output_file_name = input_file_name.replace("input", "output")

    if with_files:
        input_content = get_input_from_file(input_file_name)
    else:
        input_content = get_input_form_stdin()

    output = ""
    num_test_cases = int(input_content[0])

    for i in range(1, num_test_cases * 2, 2):
        index = int(((i + 1) / 2))
        num_substrings = int(input_content[i])
        sub_string_list = input_content[i + 1].split()
        is_possible, solution = resolve_case(index, num_substrings, sub_string_list)
        output += "Case #" + str(index) + ": "
        if is_possible:
            output += "POSSIBLE"
            output += "\n"
            for item in solution:
                output += item + " "
            output = output.strip()
        else:
            output += "IMPOSSIBLE"
        output += "\n"


    if with_files:
        output_file = open(output_file_name, 'w')
        output_file.writelines(output)
        output_file.close()
        print(output)
    else:
        print(output)