letters tower

// Those who cannot remember the past are
// condemned to repeat it (use DP -_-)
// - George Santayana

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define ll long long
#define ull unsigned long long
#define MOD 1000000007
#define PI 3.14159265
#define ceil(a, b) (((a) / (b)) + ((a) % (b) ? 1 : 0))
#define imin INT_MIN
#define imax INT_MAX
#define llmax LLONG_MAX
#define llmin LLONG_MIN
#define inf 2000000000
#define nl '\n'
#define ppcnt __builtin_popcount
#define ppcntll __builtin_popcountll
#define clz __builtin_clz
#define clzll __builtin_clzll
#define ctz __builtin_ctz
#define ctzll __builtin_ctzll
#define modulo(a, b, mod) ((((a) % (mod)) + ((b) % (mod))) % (mod))
#define cnte(v, x) count(all(v), (x))
#define mine(v) min_element(all(v))
#define maxe(v) max_element(all(v))
#define updmin(a, b) a = min(a, b)
#define updmax(a, b) a = max(a, b)
#define finmod(x, m) x = ((x) % (m) + (m)) % m
#define debug(x) cout << "x: " << (x) << nl;
#define debug2(x, y) cout << "x: " << (x) << " y: " << y << nl;
#define ordered_set tree<pair<int, int>, null_type, less<>, rb_tree_tag, tree_order_statistics_node_update>
#define ordered_map tree<int, int, less<>, rb_tree_tag, tree_order_statistics_node_update>

//vector<int> dx = {0, 0, 1, -1, 1, 1, -1, -1}, dy = {1, -1, 0, 0, 1, -1, 1, -1};
//vector<int> dx = {0, 0, 1, -1}, dy = {1, -1, 0, 0};

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

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

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

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

void Start_Crushing() {
    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
}

// remove the files before you upload your solution

void solve(){
    int n; cin >> n;
    vector<string> v(n); cin >> v;
    bitset<26> st, ed, md;
    map<char, int> same;


    for(auto& i : v){

        set<char> s(all(i));
        if(s.size() == 1) {
            same[i.front()]++;
            continue;
        }

        for(char letter = 'A'; letter <= 'Z'; letter++){
            int cnt = 0, seg = 0;

            for(auto& c : i){
                if(c != letter){
                    seg += (cnt > 0);
                    cnt = 0;
                }else
                    cnt += c == letter;
            }
            seg += cnt > 0;

            if(seg > 1) {
                return void(cout << "IMPOSSIBLE");
            }
        }

        if(st[i.front() - 'A']) {
            return void(cout << "IMPOSSIBLE");
        }
        if(ed[i.back() - 'A']) {
            return void(cout << "IMPOSSIBLE");
        }

        st[i.front() - 'A'] = true;
        ed[i.back() - 'A'] = true;

        bitset<26> tmp;
        for(int idx = 1; idx < i.size() - 1; idx++){
            if(i[idx] != i.front() and i[idx] != i.back())
                tmp[i[idx] - 'A'] = true;
        }

        for(int idx = 0; idx < 26; idx++){
            if(tmp[idx] and md[idx])
                return void(cout << "IMPOSSIBLE");

            md[idx] = tmp[idx];
        }
    }

    deque<string> ans;
    map<string, int> freq;
    for(auto& i : v){
        freq[i]++;
    }
    ans.emplace_back(v.back());
    freq[v.back()]--;

    for(int cnt = 0; cnt < n; cnt++){
        bool ok = false;

        for(auto& i : v){
            if(not freq[i])
                continue;
            set<char> s(all(i));
            if(s.size() == 1 and i.front() == ans.back().back()){
                freq[i]--;
                ans.emplace_back(i);
                same[i.front()]--;
                ok = true;
            }
            if(freq[i] and s.size() == 1 and i.back() == ans.front().front()){
                freq[i]--;
                ans.emplace_front(i);
                same[i.front()]--;
                ok = true;
            }
        }

        for(auto& i : v){
            if(freq[i] and i.front() == ans.back().back()){
                freq[i]--;
                ans.emplace_back(i);
                ok = true;
            }
            if(freq[i] and i.back() == ans.front().front()){
                freq[i]--;
                ans.emplace_front(i);
                ok = true;
            }
        }
        if(ok)
            continue;

        for(auto& i : v){
            if(freq[i] and ed[i.front()]){
                ans.emplace_back(i);
                freq[i]--;
                ok = true;
                break;
            }
            if(freq[i] and st[i.back()]){
                ans.emplace_front(i);
                freq[i]--;
                ok = true;
                break;
            }
        }
        if(ok)
            continue;

        for(auto& i : v){
            set<char> s(all(i));
            if(freq[i] and s.size() != 1 and same[i.back()]){
                freq[i]--;
                ok = true;
                ans.emplace_back(i);
            }
        }
        if(ok)
            continue;

        for(auto& i : v){
            set<char> s(all(i));
            if(freq[i] and s.size() != 1 and same[i.front()]){
                freq[i]--;
                ok = true;
                ans.emplace_front(i);
            }
        }
        if(ok)
            continue;

        for(auto& i : v){
            if(freq[i]) {
                ans.emplace_back(i);
                freq[i]--;
            }
        }
    }


    string res;
    for(auto& i : ans)
        res += i;
    for(char letter = 'A'; letter <= 'Z'; letter++){
        int cnt = 0, seg = 0;

        for(auto& c : res){
            if(c != letter){
                seg += (cnt > 0);
                cnt = 0;
            }else
                cnt += c == letter;
        }
        seg += cnt > 0;

        if(seg > 1) {
            return void(cout << "IMPOSSIBLE");
        }
    }
    cout << res;
}

int main(){
    Start_Crushing();

    int t = 1;
    /*Multiple test cases?*/ cin >> t;
//    while (t--) {
//        solve();
//        if(!t)
//            break;
//        cout << nl;
//    }

    for(int tc = 1; tc <= t; tc++){
        cout << "Case #" << tc << ": ";
        solve();
        if(tc != t)
            cout << nl;
    }

    return 0;
}