#pragma GCC optimize(2)
#include <cstdio>
#include <string>
#include <vector>
namespace iobuff {
  const int LEN = 1000000;
  char in[LEN + 5], out[LEN + 5];
  char *pin = in, *pout = out, *ed = in, *eout = out + LEN;
  inline char gc(void) {
    return pin == ed && (ed = (pin = in) + fread(in, 1, LEN, stdin), ed == in) ? EOF : *pin++;
  }
  inline void pc(char c) {
    pout == eout && (fwrite(out, 1, LEN, stdout), pout = out);
    (*pout++) = c;
  }
  inline void ps(const std::string &s) {
    for (auto c : s)
      pc(c);
  }
  inline void flush() { fwrite(out, 1, pout - out, stdout), pout = out; }
  template <typename T>
  inline void scan(T &x) {
    static int f;
    static char c;
    c = gc(), f = 1, x = 0;
    while (c < '0' || c > '9')
      f = (c == '-' ? -1 : 1), c = gc();
    while (c >= '0' && c <= '9')
      x = 10 * x + c - '0', c = gc();
    x *= f;
  }
  template <typename T>
  inline void putint(T x, char div) {
    static char s[100];
    static int top;
    top = 0;
    x < 0 ? pc('-'), x = -x : 0;
    while (x)
      s[top++] = x % 10, x /= 10;
    !top ? pc('0'), 0 : 0;
    while (top--)
      pc(s[top] + '0');
    pc(div);
  }
}
using namespace iobuff;
int main() {
  int T;
  scan(T);
  for (int TC = 1; TC <= T; TC++) {
    ps("Case #"), putint(TC, ':'), pc(' ');
    int n, m;
    scan(n), scan(m);
    std::vector<std::vector<int>> ed(n), rved(n);
    for (int i = 0; i < m; i++) {
      int u, v;
      scan(u), scan(v);
      ed[--u].push_back(--v);
      rved[v].push_back(u);
    }
    std::vector<bool> con1(n), con2(n);
    bool imp = 0;
    int lst;
    auto scc1 = [&](const auto &scc1, int u) -> void {
      con1[u] = 1;
      for (auto v : ed[u])
        if (!con1[v])
          scc1(scc1, v);
      lst = u;
    };
    scc1(scc1, 0);
    for (int i = 0; i < n; i++) {
      if (!con1[i]) {
        imp = 1;
        break;
      }
    }
    auto scc2 = [&](const auto &scc2, int u) -> void {
      con2[u] = 1;
      for (auto v : rved[u])
        if (!con2[v])
          scc2(scc2, v);
    };
    scc2(scc2, lst);
    for (int i = 0; i < n; i++) {
      if (!con2[i]) {
        imp = 1;
        break;
      }
    }
    if (imp) {
      ps("IMPOSSIBLE\n");
      continue;
    }
    std::vector<int> lnk(n, -1), vis(n, 0);
    auto dfs = [&](const auto &dfs, const int &u, std::vector<bool> &vis) -> bool {
      for (int v : ed[u]) {
        if (vis[v])
          continue;
        vis[v] = 1;
        if (!~lnk[v] || dfs(dfs, lnk[v], vis)) {
          lnk[v] = u;
          return true;
        }
      }
      return false;
    };
    int cnt = 0;
    for (int i = 0; i < n; i++) {
      std::vector<bool> vis(n, 0);
      if (dfs(dfs, i, vis))
        cnt++;
    }
    if (cnt < n) {
      ps("IMPOSSIBLE\n");
      continue;
    }
    // begin
    std::vector<std::vector<int>> nwed(n);
    for (int u = 0; u < n; u++) {
      if (imp)
        break;
      for (auto v : ed[u]) {
        if (lnk[v] == u) {
          for (int i = 0; i < n; i++) {
            nwed[lnk[i]].push_back(i);
          }
          continue;
        }
        for (int i = 0; i < n; i++)
          if (lnk[i] == u)
            lnk[i] = -1;
        int tmp = lnk[v];
        lnk[v] = u;
        std::vector<bool> vis(n, 0);
        vis[v] = 1;
        if (!dfs(dfs, tmp, vis)) {
          imp = 1;
          break;
        }
        for (int i = 0; i < n; i++) {
          nwed[lnk[i]].push_back(i);
        }
      }
    }
    if (imp) {
      ps("IMPOSSIBLE\n");
      continue;
    }
    // end
    putint( n * m + 1 , '\n');
    std::vector<int> ans;
    auto fnd = [&](const auto &fnd, const int &u) -> void {
      for (auto &x : nwed[u]) {
        int v = x;
        if (!~v)
          continue;
        x = -1;
        fnd(fnd, v);
        ans.push_back(u);
      }
    };
    fnd(fnd, 0);
    int st = 0;
    for (; ans[st] != 0; st++)
      ;
    for (int i = st; ~i; i--) {
      putint(ans[i]+1, ' ');
    }
    for (int i = n * m - 1; i > st; i--) {
      putint(ans[i]+1, ' ');
    }
    ps("1\n");
  }
  flush();
}