import sys
input = sys.stdin.readline

from collections import Counter

T=int(input())
for tests in range(T):
    N=int(input())
    S=input().split()

    flag=1

    for i in range(N):
        if i==0:
            S[i]="".join(sorted(S[i]))
            continue

        Y=list(S[i-1])
        Z=sorted(S[i],reverse=True)

        if Y>Z:
            flag=0
            break
        X=[]

        C=Counter(S[i])

        for j in range(len(S[i])):
            flag2=0
            if j<len(Y) and C[Y[j]]>0:
                X+=[Y[j]]
                C[Y[j]]-=1

                XX=[]

                for a in "ABCDEFGHIJKLMNOPQRSTUVWXYZ"[::-1]:
                    XX+=a*C[a]

                if X+XX>=Y:
                    flag2=1
                else:
                    C[Y[j]]+=1
                    X.pop()
                    

            if flag2==0:
                if j<len(Y):
                    for a in "ABCDEFGHIJKLMNOPQRSTUVWXYZ":
                        if a>Y[j] and C[a]>0:
                            X+=[a]
                            C[a]-=1
                            break
                for a in "ABCDEFGHIJKLMNOPQRSTUVWXYZ":
                    X+=a*C[a]

                break

        S[i]="".join(X)
                
                
                    
                    
                    
            
        

    #print(S)

    if flag==0:
        ANS="IMPOSSIBLE"
    else:
        ANS="POSSIBLE\n"+" ".join(S)

    print("Case #"+str(tests+1)+": "+str(ANS))