#!/usr/bin/env python3

from collections import Counter

def solve():
    p = int(input())
    w = input().split()

    res = do_solve(w)
    if res is None:
        return 'IMPOSSIBLE'
    return f'POSSIBLE\n{" ".join(res)}'


def do_solve(w):
    last = ''.join(sorted(w[0]))
    res = [last]
    for letters in w[1:]:
        nw = make_word(last, letters)
        if nw is None:
            return None
        res.append(nw)
        last = nw
    return res


def make_word(prev, letters):
    letters = Counter(letters)
    sb = []
    backup = None
    for c in prev:
        nc = min((k for k, v in letters.items() if v > 0 and k > c), default=None)
        if nc is not None:
            bl = dict(letters)
            bl[nc] -= 1
            backup = [*sb, nc], bl
        if letters.get(c, 0) > 0:
            sb.append(c)
            letters[c] -= 1
        else:
            if backup is None:
                return None
            sb, letters = backup
            break
    sb.extend(sorted(k * v for k, v in letters.items()))
    return ''.join(sb)


if __name__ == '__main__':
    t = int(input())
    for i in range(t):
        print(f'Case #{i + 1}: {solve()}')