T = int(input())

test = 1

def sort1st(v):
    return v[0]

def sortnum(n):
    return sort1st(sch[n])


ANS = []
while test <= T:
    N = int(input())

    sch = []

    ev = []
    J = []
    C = []
    num = []
    chart = []
    sollist = []
    for n in range(0, N):
        ev = input().split()
        for i in ev:
            ev[ev.index(i)] = int(i)
        sch.append(ev)
        num.append(n)
        sollist.append("")







    for x in range(0, 1450):
        chart.append(0)
        J.append(0)
        C.append(0)


    num.sort(key = sortnum)
    sch.sort(key = sort1st)






    for v in sch:

        for m in range(v[0], v[1]):
            chart[m] += 1

    isimp = False

    for m in chart:
        if m > 2:
            isimp = True
            sol = "IMPOSSIBLE"
            break


    if isimp == False:
        sol = ""
        for v in range(0, N):
            jbusy = False
            cbusy = False
            for m in range(sch[v][0], sch[v][1]):
                if J[m] != 0:

                    jbusy = True
                if C[m] != 0:

                    cbusy = True

            if cbusy == False:
                sollist[v] = "C"
                for m in range(sch[v][0], sch[v][1]):
                    C[m] += 1


            elif jbusy == False:
                sollist[v] = "J"
                for m in range(sch[v][0], sch[v][1]):
                    J[m] += 1





        for nchar in range(0, len(sollist)):
            sol += sollist[num[nchar]]
    ANS.append([str(test), sol])
    test += 1

for u in ANS:
    print("Case #" + u[0] + ":", u[1])