from collections import Counter
import string


def defeat(prev: str, token: str):
    "min sort of token beating prev, or None"

    ctr = Counter(token)
    best = None
    for n_shared in range(len(prev)):
        pool = [c for c, v in ctr.items() if c > prev[n_shared] and v > 0]
        if pool:
            c = min(pool)
            ctr[c] -= 1
            best = prev[:n_shared] + c + ''.join(sorted([c * v for c, v in ctr.items()]))
            ctr[c] += 1

        # end
        if ctr[prev[n_shared]] > 0:
            ctr[prev[n_shared]] -= 1
        else:
            return best
    # share all
    best = prev + ''.join(sorted([c*v for c, v in ctr.items()]))
    return best


def solve(tokens):
    ret = []
    for token in tokens:
        prev = ret[-1] if ret else ""
        res = defeat(prev, token)
        if res is None: return None
        ret.append(res)
    return ret

tcn = int(input())
for tc in range(1, tcn+1):
    input()
    tokens = input().split()
    res = solve(tokens)
    verdict = "IMPOSSIBLE" if res is None else "POSSIBLE"
    print("Case #{}: {}".format(tc, verdict))
    if res is not None:
        print(*res)