#include <bits/stdc++.h>
#define db double
#define re register
#define il inline
#define ll long long
#define ull unsigned ll
#define i128 __int128
#define pii pair<int, int>
#define fi first
#define se second
#define eb emplace_back
#define SZ(v) (int)v.size()
#define ALL(v) v.begin(), v.end()
#define mems(v, x) memset(v, x, sizeof(v))
#define FOR(i, L, R) for(re int i = (L); i <= (R); ++i)
#define ROF(i, R, L) for(re int i = (R); i >= (L); --i)
#define LS i << 1, l, mid
#define RS i << 1 | 1, mid + 1, r
using namespace std;
#define N 405
#define M 5005
int n, m, u[M], v[M], mt[N];
vector<int> e[N]; bool vis[N];
il bool dfs(int x) {
  for(auto y : e[x]) if(!vis[y]) {
    vis[y] = 1; if(!mt[y] || dfs(mt[y])) return mt[y] = x, mt[x] = y, 1;
  }
  return 0;
}
namespace euler {
  vector<int> e[N], ns;
  il void add(int x, int y) {e[x].eb(y);}
  il void clr() {FOR(i, 1, n) vector<int>().swap(e[i]);}
  il void dfs(int x) {
    while(!e[x].empty()) {
      int y = e[x].back(); e[x].pop_back(); dfs(y);
    }
    ns.eb(x);
  }
  il void solve() {
    vector<int>().swap(ns);
    dfs(1), reverse(ALL(ns));
    FOR(i, 1, n) if(!e[i].empty()) {cout << "IMPOSSIBLE\n"; return ;}
    cout << SZ(ns) << "\n";
    for(auto x : ns) cout << x << " ";
    cout << "\n";
  }
}
il void WORK(int TC) {
  cout << "Case #" << TC << ": ";
  cin >> n >> m;
  FOR(i, 1, n * 2)
    vector<int>().swap(e[i]), mt[i] = 0;
  FOR(i, 1, m) cin >> u[i] >> v[i], e[u[i]].eb(v[i] + n);
  FOR(i, 1, n) {
    mems(vis, 0);
    if(!dfs(i)) {cout << "IMPOSSIBLE\n"; return ;}
  }
  // cerr << "ok\n";
  // FOR(i, 1, n) cerr << i << " " << mt[i] << "\n";
  euler::clr();
  FOR(i, 1, m) {
    int x = u[i], y = v[i] + n;
    if(mt[x] != y) {
      int p = mt[y];
      mems(vis, 0), vis[y] = 1;
      mt[mt[x]] = mt[p] = 0;
      mt[x] = y, mt[y] = x;
      if(!dfs(p)) {cout << "IMPOSSIBLE\n"; return ;}
    }
    FOR(u, 1, n) if(mt[u] <= n) while(1);
    FOR(u, 1, n) euler::add(u, mt[u] - n);
  }
  // cerr << "ok\n";
  euler::solve();
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
  int T, TC = 0; cin >> T;
  while(T--) WORK(++TC);
  cerr << 1.0 * clock() / CLOCKS_PER_SEC;
  return 0;
}