#include <bits/stdc++.h>
using namespace std;
const int N = 5e2 + 5,M = 4e4 + 5;
int n,m;
int U[M],V[M];
int fir[N],nxt[M],st[M],to[M],ect = 1;
inline void addedge(int u1,int v1) {
	nxt[++ect] = fir[u1];fir[u1] = ect;to[ect] = v1;st[ect] = u1;
}
bool vis[N];
int mth[N],mthe[N];
bool dfs(int x) {
	for(int i = fir[x],y;y = to[i],i;i = nxt[i])
		if(!vis[y]) {
			vis[y] = 1;
			if(!mth[y] || dfs(mth[y])) { mth[x] = y;mth[y] = x;mthe[x] = i;return 1;}
		}
	return 0;
}
int pv[N],pe[N];
void Bfs(int st) {
	queue<int> Q;
	for(int i = 1;i <= n + n;i++) vis[i] = pv[i] = pe[i] = 0;
	Q.push(st);vis[st] = true;
	while(!Q.empty()) {
		int x = Q.front();Q.pop();
		// printf("st,x:%d,%d\n",st,x);
		if(x > n) {
			int y = mth[x];if(vis[y]) continue;
			pv[y] = x;pe[y] = mthe[y];
			Q.push(y);vis[y] = true;
		} else 
			for(int i = fir[x],y;y = to[i],i;i = nxt[i])
				if(i != mthe[x] && !vis[y]) {
					vis[y] = true;
					pv[y] = x;pe[y] = i;
					Q.push(y);
				} 
	}
}
vector<int> Ans;
namespace Euler {
	const int N = 1e3 + 5,M = 4e6 + 5;
	int fir[N],nxt[M],to[M],ect = 1,sgn[M];
	inline void addvir(int u1,int v1) {
		// printf("vir:%d->%d\n",u1,v1);
		nxt[++ect] = fir[u1];fir[u1] = ect;to[ect] = v1;sgn[ect] = 0;
	}
	void Euler(int x) {
		for(int i = fir[x],y;y = to[i],i;i = fir[x]) {
			fir[x] = nxt[i];
			if(sgn[i]) continue;
			sgn[i] =  true;
			Euler(y);
		}
		Ans.push_back(x);
	}
	void Clr(int n) {
		ect = 1;
		for(int i = 1;i <= n;i++) fir[i] = 0;
		Ans.clear();
	}
}
bool inmth[M];
void work() {
	cin >> n >> m;
	for(int i = 1;i <= m;i++) cin >> U[i] >> V[i];
	for(int i = 1;i <= n + n;i++) 
		pv[i] = pe[i] = mth[i] = vis[i] = mthe[i] = fir[i] = 0;
	Euler::Clr(n);
	ect = 1;
	for(int i = 1;i <= m;i++)
		addedge(U[i],V[i] + n);
	for(int i = 1;i <= n;i++) {
		for(int j = 1;j <= n + n;j++) vis[j] = 0;
		if(!dfs(i)) return puts("IMPOSSIBLE"),void();
	}
	for(int i = 1;i <= ect;i++) inmth[i] = 0;
	for(int i = 1;i <= n;i++) inmth[mthe[i]] = true,Euler::addvir(i,mth[i] - n);
	for(int i = 1;i <= m;i++) {
		int u = U[i],v = V[i] + n;
		if(mth[u] != v) {
			mth[mth[u]] = 0;
			for(int j = 1;j <= n + n;j++) vis[j] = 0;
			vis[v] = 1;
			if(!dfs(mth[v])) return puts("IMPOSSIBLE"),void();
			else mth[u] = v,mth[v] = u;
		} 
		for(int j = 1;j <= n;j++)
			Euler::addvir(j,mth[j] - n);
	}
	// for(int i = 1;i <= n;i++) {
	// 	Bfs(n + i);
	// 	for(int j = 1;j <= ect;j++)
	// 		if(!inmth[j] && to[j] == n + i) {
	// 			int u = st[j],v = n + i;
	// 			if(!vis[u]) return puts("IMPOSSIBLE"),void();
	// 			int now = u;
	// 			while(now != v) inmth[pe[now]] ^= 1,now = pv[now];
	// 			inmth[j] ^= 1;
	// 			for(int k = 1;k <= ect;k++)
	// 				if(inmth[k]) Euler::addvir(st[k],to[k] - n);
	// 			now = u;
	// 			while(now != v) inmth[pe[now]] ^= 1,now = pv[now];
	// 			inmth[j] ^= 1;
	// 		}
	// }
	Euler::Euler(1);
	if(Ans.size() != Euler::ect) return puts("IMPOSSIBLE"),void(); 
	printf("%d\n",(int)Ans.size());
	reverse(Ans.begin(),Ans.end());
	// Ans.push_back(1);
	for(auto i : Ans) printf("%d ",i);
	printf("\n");
}
int main() {
	int T;
	cin >> T;
	for(int _ = 1;_ <= T;_++) {
		printf("Case #%d: ",_);
		work();
	}
	return 0;
}

/*
1
3 6
1 2
1 3
2 1
2 3
3 1
3 2
*/