from bisect import bisect_left

def answer(S):
    """ at most 100 of 100-character strings """
    result = ["".join(sorted(S[0]))]
    for s in S[1:]:
        result.append(earliestPermutationAfter(s, result[-1]))
        if not result[-1]: return False, False
    return True, result
    
def earliestPermutationAfter(s, target):
    if len(target) > len(s):
        target = target[:len(s)-1] + chr(ord(target[len(s)])+1)
    s = sorted(list(s))
    result = []
    for c in target:
        i = bisect_left(s, c)
        try:
            result.append(s[i])
            s = s[:i]+s[i+1:]
            if result[-1] != c: return "".join(result+s)
        except:
            return False
    return "".join(result+s)

T = int(input()) # read a line with a single integer
for i in range(1, T + 1):
    P = int(input())
    S = input().split(" ")
    poss, ans = answer(S)
    print(f'Case #{i}: {"POSSIBLE" if poss else "IMPOSSIBLE"}')
    if poss: print(" ".join(ans))