#include<bits/stdc++.h>
using namespace std;
namespace my_std{
	#define ll long long
	#define bl bool
	ll my_pow(ll a,ll b,ll mod){
		ll res=1;
		if(!b) return 1;
		while(b){
			if(b&1) res=(res*a)%mod;
			a=(a*a)%mod;
			b>>=1;
		}
		return res;
	}
	ll qpow(ll a,ll b){
		ll res=1;
		if(!b) return 1;
		while(b){
			if(b&1) res*=a;
			a*=a;
			b>>=1;
		}
		return res;
	}
	#define db double
	#define pf printf
	#define pc putchar
	#define fr(i,x,y) for(register ll i=(x);i<=(y);i++)
	#define pfr(i,x,y) for(register ll i=(x);i>=(y);i--)
	#define go(u) for(ll i=head[u];i;i=e[i].nxt)
	#define enter pc('\n')
	#define space pc(' ')
	#define fir first
	#define sec second
	#define MP make_pair
	#define il inline
	#define inf 8e18
	#define random(x) rand()*rand()%(x)
	#define inv(a,mod) my_pow((a),(mod-2),(mod))
	il ll read(){
		ll sum=0,f=1;
		char ch=0;
		while(!isdigit(ch)){
			if(ch=='-') f=-1;
			ch=getchar();
		}
		while(isdigit(ch)){
			sum=sum*10+(ch^48);
			ch=getchar();
		}
		return sum*f;
	}
	il void write(ll x){
		if(x<0){
			x=-x;
			pc('-');
		}
		if(x>9) write(x/10);
		pc(x%10+'0');
	}
	il void writeln(ll x){
		write(x);
		enter;
	}
	il void writesp(ll x){
		write(x);
		space;
	}
}
using namespace my_std;
vector<ll> vec[220],ans;
ll t,n,m,to[220],head[220],cnt=0;
bl ck[220];
struct node{
	ll nxt,to;
}e[1000010];
void add(ll u,ll v){
	e[++cnt].nxt=head[u];
	e[cnt].to=v;
	head[u]=cnt;
}
bl match(ll u){
	fr(i,0,(ll)vec[u].size()-1){
		ll v=vec[u][i];
		if(!ck[v]){
			ck[v]=1;
			if(!to[v]||match(to[v])){
				to[v]=u;
				return 1;
			}
		}
	}
	return 0;
}
void dfs(ll u){
	ck[u]=1;
	for(ll i=head[u];i;i=head[u]){
		head[u]=e[i].nxt;
		dfs(e[i].to);
	}
	ans.push_back(u);
}
void clr(){
	fr(i,1,n) vec[i].clear();
	ans.clear();
	fr(i,1,n) head[i]=to[i]=0;
	cnt=0;
	fr(i,1,n) ck[i]=0;
}
int main(){
	t=read();
	fr(_,1,t){
		n=read();
		m=read();
		if(_==25) pf("%lld %lld\n",n,m);
		fr(i,1,m){
			ll u=read(),v=read();
			if(_==25) pf("%lld %lld\n",u,v);
			vec[u].push_back(v);
		}
		if(_!=25){
			clr();
			continue;
		}
		bl pd=0;
		fr(i,1,n){
			fr(j,1,n) ck[j]=0;
			if(!match(i)) pd=1;
		}
		fr(u,1,n){
			fr(i,0,(ll)vec[u].size()-1){
				ll v=vec[u][i];
				if(to[v]!=u){
					fr(j,1,n) if(to[j]==u) to[j]=0;
					ll w=to[v];
					to[v]=u;
					fr(j,1,n) ck[j]=0;
					ck[v]=1;
					if(!match(w)) pd=1;
				}
				fr(j,1,n) add(to[j],j);
			}
		}
		pf("Case #%lld: ",_);
		if(pd){
			pf("IMPOSSIBLE\n");
			clr();
			continue;
		}
		fr(i,1,n) ck[i]=0;
		dfs(1);
		writeln(ans.size());
		pfr(i,(ll)ans.size()-1,0) writesp(ans[i]);
		enter;
		clr();
	}
}