#include <cstdio>
#include <algorithm>
#include <vector>
#include <cstring>
using namespace std;
namespace fr{
	#include <ctype.h>
	char buf[1<<23],*p1=buf,*p2=buf,obuf[1<<23],*O=obuf;
	#define getchar() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++)
	#define putchar(c) (((O==obuf+(1<<23))?(fwrite(obuf,1,O-obuf,stdout),O=obuf):O),*O++=(c))
	void rd(char *x){char c=getchar();while(isspace(c)) c=getchar();while(!isspace(c) && c!=EOF) *(x++)=c,c=getchar();*(x++)='\0';}
	template<class T> void rd(T &x){T ret=0;bool flg=false;char c=getchar();while(!isdigit(c) && c!='-') c=getchar();
		if(c=='-') c=getchar(),flg=true;while(isdigit(c)) ret=(ret<<1)+(ret<<3)+(c-'0'),c=getchar();if(flg) x=-ret;else x=ret;}
	void pc(char c){putchar(c);}void flush(){fwrite(obuf,1,O-obuf,stdout),O=obuf;}
	template<class T> void wr(T x){if(x<0) putchar('-'),x=-x;if(x==0) putchar('0');static int cnt,stk[110];cnt=0;while(x) stk[++cnt]=x%10,x/=10;for(int i=cnt;i;--i) putchar(stk[i]+'0');}
}
using fr::rd;using fr::pc;using fr::wr;using fr::flush;
constexpr int N=210,M=5010,inf=1e9;
vector<int> es[N];
int eds[M][3],n,m,ind[N],otd[N];
bool vis[N];
void clear()
{
	memset(vis,0,sizeof(vis));
	memset(ind,0,sizeof(ind));
	memset(otd,0,sizeof(otd));
	for(int i(1);i<=n;++i)
		es[i].clear();
}
void dfs(int u)
{
	vis[u]=true;
	for(int v:es[u])
	if(!vis[v])
		dfs(v);
}
namespace flow
{
	constexpr int N=410,M=5010+(N<<1);
	int hd[N],nxt[M<<1],cur[N],tv[M<<1],ecnt=1,tag[N],w[M<<1];
	void clear()
	{
		ecnt=1;
		memset(hd,0,sizeof(hd));
	}
	int add_edge(int u,int v,int c)
	{
		nxt[++ecnt]=hd[u];
		tv[ecnt]=v;
		hd[u]=ecnt;
		w[ecnt]=c;
		return ecnt;
	}
	int bfs(int s,int t)
	{
		memset(tag,0,sizeof(tag));
		static int qq[N];int l(1),r(1);
		qq[r++]=s;tag[s]=1;
		while(l<r)
		{
			int u=qq[l++];
			for(int p(hd[u]);p;p=nxt[p]) if(w[p] && !tag[tv[p]])
			{
				tag[tv[p]]=tag[u]+1;
				qq[r++]=tv[p];
			}
		}
		return tag[t]!=0;
	}
	int dfs(int u,int t,int fl)
	{
		if(u==t) return fl;
		int ret(0);
		for(int (&p)(cur[u]);p;p=nxt[p])
		if(w[p] && tag[tv[p]]==tag[u]+1)
		{
			int k=dfs(tv[p],t,min(w[p],fl));
			fl-=k;ret+=k;
			w[p]-=k;w[p^1]+=k;
			if(!fl)
				break;
		}
		return ret;
	}
	int dinic(int s,int t)
	{
		int ans(0);
		while(bfs(s,t))
		{
			memcpy(cur,hd,sizeof(cur));
			ans+=dfs(s,t,inf);
		}
		return ans;
	}
}
namespace euler_cycle
{
	constexpr int N=1000010;
	int hd[N],nxt[N],tv[N],ecnt;
	void clear()
	{
		ecnt=0;
		memset(hd,0,sizeof(hd));
	}
	void add_edge(int u,int v)
	{
		// printf("u %d v %d\n",u,v);
		nxt[++ecnt]=hd[u];
		tv[ecnt]=v;
		hd[u]=ecnt;
	}
	void dfs(int u,vector<int> &ans)
	{
		for(int (&p)(hd[u]);p;)
		{
			int v=tv[p];
			p=nxt[p];
			dfs(v,ans);
		}
		ans.push_back(u);
	}
	vector<int> calc(int s=1)
	{
		vector<int> ret;
		dfs(s,ret);
		reverse(ret.begin(),ret.end());
		return ret;
	}
}
int calc(int k)
{
	int s(n*2+1),t(s+1);
	flow::clear();
	int tot(0);
	for(int i(1);i<=n;++i)
	{
		flow::add_edge(s,i,k-otd[i]);
		flow::add_edge(i,s,0);
		flow::add_edge(i+n,t,k-ind[i]);
		flow::add_edge(t,i+n,0);
		tot+=(k-ind[i]);
	}
	for(int i(1);i<=m;++i)
	{
		flow::add_edge(eds[i][0],eds[i][1]+n,inf);
		eds[i][2]=flow::add_edge(eds[i][1]+n,eds[i][0],0);
	}
	return flow::dinic(s,t)-tot;
}
int main()
{
	int TC;
	rd(TC);
	for(int ti(1);ti<=TC;++ti)
	{
		pc('C');pc('a');pc('s');pc('e');pc(' ');pc('#');wr(ti);pc(':');pc(' ');
		rd(n);rd(m);
		for(int i(1),u,v;i<=m;++i)
		{
			rd(u);rd(v);
			es[u].push_back(v);
			++ind[v];++otd[u];
			eds[i][0]=u;eds[i][1]=v;
		}
		dfs(1);
		bool ok(true);
		for(int i(1);i<=n && ok;++i)
		if(!vis[i])
			ok=false;
		if(!ok)
		{
			for(char c:"IMPOSSIBLE\n")
			if(c!='\0')
				pc(c);
			clear();
			continue;
		}
		int ans(1000000/n);
		int k(calc(ans));
		if(k!=0)
		{
			for(char c:"IMPOSSIBLE\n")
			if(c!='\0')
				pc(c);
			clear();
			continue;
		}
		euler_cycle::clear();
		for(int i(1);i<=m;++i)
		{
			int c=flow::w[eds[i][2]]+1;
			for(int j(1);j<=c;++j)
				euler_cycle::add_edge(eds[i][0],eds[i][1]);
		}
		auto ap=euler_cycle::calc();
		int l=ap.size();
		wr(l);pc('\n');
		for(int i(0);i<l;++i)
			wr(ap[i]),pc(" \n"[i==l-1]);
		clear();
	}
	flush();
	return 0;
}