/* name: 4513
 * author: 5ab
 * created at: 2024-03-29
 */
#include <bits/stdc++.h>
using namespace std;

#define all(x) (x).begin(), (x).end()
#define ssz(x) (int((x).size()))

auto chmax = [](auto& x, auto y) { if (x < y) x = y; };
auto chmin = [](auto& x, auto y) { if (y < x) x = y; };

using ll = long long;
const int N = 200, M = 5000, ND = 2 * N + 2, E = 2 * N + M, INF = 1e9;

int hd[ND], des[E * 2], nxt[E * 2], val[E * 2], dis[ND], pre[ND], ec = 0;
bool ok[M], vs[N];

void _add(int s, int t, int v)
{
	des[ec] = t, val[ec] = v;
	nxt[ec] = hd[s], hd[s] = ec++;
}
void add(int s, int t, int v) {
	_add(s, t, v), _add(t, s, 0);
}

int s, t, nd;

queue<int> q;
void bfs(int sp = s)
{
	fill(dis, dis + nd, INF);
	fill(pre, pre + nd, -1);
	q.push(sp), dis[sp] = 0;
	while (!q.empty())
	{
		int cx = q.front(); q.pop();
		for (int p = hd[cx], dst; p != -1; p = nxt[p])
		{
			dst = des[p];
			if (val[p] && dis[dst] > dis[cx] + 1)
			{
				dis[dst] = dis[cx] + 1;
				pre[dst] = p;
				q.push(dst);
			}
		}
	}
}

vector<int> ng[N];
int cur[N];
vector<int> stk;

void dfs2(int id)
{
	vs[id] = 1;
	for (int &i = cur[id]; i < ssz(ng[id]); )
		dfs2(ng[id][i++]);
	stk.push_back(id);
}

signed main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	
	int cas, n, m;
	
	cin >> cas;
	for (int cid = 1; cid <= cas; cid++)
	{
		cin >> n >> m;
		s = n * 2, t = n * 2 + 1, nd = n * 2 + 2;
		
		fill(hd, hd + nd, -1), ec = 0;
		for (int i = 0, x, y; i < m; i++)
		{
			cin >> x >> y, x--, y--;
			add(x, y + n, 1);
		}
		for (int i = 0; i < n; i++)
		{
			add(s, i, 1);
			add(i + n, t, 1);
		}
		
		auto doFl = [&](int p) {
			// cerr << "fl " << p << endl;
			val[p]--, val[p ^ 1]++;
		};
		auto doBan2 = [&](int p) {
			p *= 2;
			val[p] = 0, val[p ^ 1] = 0;
		};
		auto doOk = [&](int p) {
			p *= 2;
			val[p] = 0, val[p ^ 1] = 1;
		};
		auto doInv = [&](int p) {
			// cerr << "inv " << p << endl;
			val[p]++, val[p ^ 1]--;
		};
		auto doInv2 = [&](int p) { doInv(p * 2); };
		
		int fl = 0;
		auto aug = [&]() -> bool
		{
			bfs();
			if (pre[t] == -1)
			{
				// cerr << "aug failed" << endl;
				return 0;
			}
			for (int x = t; x != s; x = des[pre[x] ^ 1])
				doFl(pre[x]);
			fl++;
			return 1;
		};
		while (aug());
		
		if (fl != n)
		{
			cout << "Case #" << cid << ": IMPOSSIBLE\n";
			continue;
		}
		
		for (int i = 0; i < n; i++)
			ng[i].clear();
		
		fill(ok, ok + m, 0);
		bool sol = 1;
		for (int i = 0; i < m; i++) if (!ok[i])
		{
			int x = des[i * 2 + 1], y = des[i * 2] - n;
			// cerr << x << " " << y << endl;
			for (int p = hd[x]; p != -1; p = nxt[p])
				if (!val[p] && des[p] >= n)
				{
					doInv(p), doInv2(m + x * 2), doInv2(m + (des[p] - n) * 2 + 1);
					fl--;
				}
			for (int p = hd[y + n]; p != -1; p = nxt[p])
				if (val[p] && des[p] < n)
				{
					doInv(p ^ 1), doInv2(m + des[p] * 2), doInv2(m + y * 2 + 1);
					fl--;
				}
			doBan2(m + x * 2), doBan2(i), doBan2(m + y * 2 + 1);
			fl++;
			// cerr << "try aug\n";
			while (aug());
			doOk(m + x * 2), doOk(i), doOk(m + y * 2 + 1);
			
			if (fl != n)
			{
				sol = 0;
				break;
			}
			for (int j = 0; j < m; j++)
				if (!val[j * 2])
				{
					ok[j] = 1;
					// cerr << des[j * 2 + 1] << " " << des[j * 2] - n << endl;
					ng[des[j * 2 + 1]].push_back(des[j * 2] - n);
				}
		}
		cout << "Case #" << cid << ": ";
		if (sol)
		{
			stk.clear();
			fill(cur, cur + n, 0);
			fill(vs, vs + n, 0);
			dfs2(0);
			for (int j = 0; j < n; j++)
				if (!vs[j])
					sol = 0;
			if (sol)
			{
				reverse(all(stk));
				// stk.push_back(0);
				cout << ssz(stk) << "\n";
				for (int x : stk)
					cout << x + 1 << " ";
				cout << "\n";
				continue;
			}
		}
		cout << "IMPOSSIBLE\n";
	}
	
	return 0;
}
// started coding at: 03-29 23:13:52