n = int(input())
cases = []
ps = {}
ps['C'] = 0
ps['J'] = 0
for i in range(n):
    a = int(input())
    day = []
    for j in range(a):
        act = list(map(int,input().split()))
        day.append(act)
    cases.append(day)
for enum,c in enumerate(cases):
    for k in ps.keys():
        ps[k] = 0
    tmp = sorted(c,key=lambda x: x[0])
    sch = [""]*len(tmp)
    for k,v in ps.items():
        if v<=tmp[0][0]:
            ps[k] = tmp[0][1]
            sch[c.index(tmp[0])] = k
            break
    for i in range(1,len(tmp)):
        if tmp[i-1][1] > tmp[i][0]:
            if all(x > tmp[i][0] for x in list(ps.values())):
                sch = "IMPOSSIBLE"
                break
            for k,v in ps.items():
                if v <= tmp[i][0]:
                    ps[k] = tmp[i][1]
                    sch[c.index(tmp[i])] = k
                    break
        elif tmp[i-1][1] <= tmp[i][0]:
            for k,v in ps.items():
                if v <= tmp[i][0]:
                    ps[k] = tmp[i][1]
                    sch[c.index(tmp[i])] = k
                    break
    print("Case #"+str(enum+1)+":","".join(sch))