#include <bits/stdc++.h>

using namespace std;

#define cin(vec) for(auto& i : vec) cin >> i
#define cin_2d(vec, n, m) for(int i = 0; i < n; i++) for(int j = 0; j < m && cin >> vec[i][j]; j++);
#define cout(vec) for(auto& i : vec) cout << i << " "; cout << "\n";
#define cout_2d(vec, n, m) for(int i = 0; i < n; i++, cout << "\n") for(int j = 0; j < m && cout << vec[i][j] << " "; j++);
#define cout_map(mp) for(auto& [f, s] : mp) cout << f << "  " << s << "\n";
#define Time cerr << "Time Taken: " << (float)clock() / CLOCKS_PER_SEC << " Secs" << "\n";
#define fixed(n) fixed << setprecision(n)
#define ceil(n, m) (((n) / (m)) + ((n) % (m) ? 1 : 0))
#define fill(vec, value) memset(vec, value, sizeof(vec));
#define Num_of_Digits(n) ((int)log10(n) + 1)
#define mod_combine(a, b, m) (((a % m) * (b % m)) % m)
#define all(vec) vec.begin(), vec.end()
#define rall(vec) vec.rbegin(), vec.rend()
#define sz(x) int(x.size())
#define debug(x) cout << #x << ": " << (x) << "\n";
#define fi first
#define se second
#define Pair pair < int, int >
#define ll long long
#define ull unsigned long long
#define Mod  1'000'000'007
#define OO 2'000'000'000
#define EPS 1e-9
#define PI acos(-1)

void AhMeD_HoSSaM(){
    ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
    #ifndef ONLINE_JUDGE
        freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
    #endif
}

template < typename T = int > istream& operator >> (istream &in, vector < T > &v) {
    for (auto &x: v) in >> x;
    return in;
}

template < typename T = int > ostream& operator << (ostream &out, const vector < T > &v) {
    for (const T &x: v) out << x << ' ';
    return out;
}

bool check(string& s){
    vector < int > freq(26, 0);
    freq[s[0] - 'A']++;
    for(int i = 1; i < sz(s); i++){
        if(s[i] == s[i - 1]) continue;
        else{
            if(freq[s[i] - 'A'] == 0) freq[s[i] - 'A']++;
            else return false;
        }
    }
    return true;
}

void Solve(){
    int n;
    cin >> n;
    vector < string > vec(n);
    cin >> vec;
    sort(all(vec));
    do{
        string tower;
        for(auto &i : vec) tower += i;
        if(check(tower)){
            cout << tower << "\n";
            return;
        }
    }while(next_permutation(all(vec)));
    cout << "IMPOSSIBLE\n";
}

int main(){
    AhMeD_HoSSaM();
    int t = 1;
    cin >> t;
    for(int i = 1; i <= t; i++){
        cout << "Case #" << i << ": ";
        Solve();
    }
    return 0;
}