func_code_string: #include <bits/stdc++.h>   using namespace std;   const int INF = 2e9 + 1;   const long long INFLL = 1e18 + 1;   struct Graph {     struct edge {       int c, f, link, to, ind;       int c2 = 0;       edge() {}       edge(int c, int f, int link, int to, int ind)           : c(c), f(f), link(link), to(to), ind(ind) {}     };     vector<vector<edge>> g;     int s, t, n;     Graph(int N, int S, int T) {       n = N;       s = S, t = T;       g.resize(n);     }     vector<int> usd;     int dfs(int v, int now) {       usd[v] = 1;       if (v == t) return now;       for (auto &e : g[v]) {         if (usd[e.to] || e.f == e.c) continue;         int x = dfs(e.to, min(now, e.c - e.f));         if (x == 0) continue;         e.f += x;         g[e.to][e.link].f -= x;         return x;       }       return 0;     }     int Ford_Fulkerson() {       int ans = 0;       while (1) {         usd.assign(n, 0);         int d = dfs(s, INF);         ans += d;         if (d == 0) break;       }       return ans;     }     void add_not_dir(int a, int b, int c, int i) {       g[a].emplace_back(c, 0, g[b].size(), b, i);       g[b].emplace_back(c, 0, g[a].size() - 1, a, i);     }     void add_dir(int a, int b, int c, int i) {       g[a].emplace_back(c, 0, g[b].size(), b, i);       g[b].emplace_back(0, 0, g[a].size() - 1, a, i);     }     int Edmonds_Karp() {       int ans = 0;       while (1) {         queue<int> q;         vector<int> p(n, -1), dist(n, INF), mn(n, INF);         q.push(s);         dist[s] = 0;         while (!q.empty()) {           int v = q.front();           q.pop();           for (auto &e : g[v]) {             if (e.f == e.c || dist[e.to] != INF) continue;             p[e.to] = v;             q.push(e.to);             dist[e.to] = dist[v] + 1;             mn[e.to] = min(mn[v], e.c - e.f);           }         }         if (dist[t] == INF) break;         vector<int> path;         int now = t;         while (now != -1) {           path.emplace_back(now);           now = p[now];         }         reverse((path).begin(), (path).end());         int F = mn[t];         ans += F;         for (int i = 0; i < (int)path.size() - 1; i++) {           int v = path[i];           for (auto &e : g[v]) {             if (e.to == path[i + 1] && e.c - e.f >= F) {               e.f += F;               g[e.to][e.link].f -= F;               break;             }           }         }       }       return ans;     }     vector<int> d, uk;     int dfs_Dinic(int v, int now) {       if (v == t || now == 0) return now;       for (int i = uk[v]; i < (int)g[v].size(); i++) {         if (d[g[v][i].to] == d[v] + 1) {           int can = g[v][i].c2 - g[v][i].f;           int x = dfs_Dinic(g[v][i].to, min(can, now));           if (x != 0) {             g[v][i].f += x;             g[g[v][i].to][g[v][i].link].f -= x;             return x;           }         }         uk[v]++;       }       return 0;     }     bool bfs() {       d.assign(n, INF);       d[s] = 0;       deque<int> q;       q.push_back(s);       while (!q.empty()) {         int v = q.front();         q.pop_front();         for (auto &e : g[v]) {           if (e.f < e.c2 && d[e.to] == INF) {             d[e.to] = d[v] + 1;             q.push_back(e.to);           }         }       }       return d[t] != INF;     }     long long Dinic() {       long long ans = 0;       while (bfs()) {         uk.assign(n, 0);         while (1) {           int d = dfs_Dinic(s, INF);           if (d == 0) break;           ans += d;         }       }       return ans;     }     long long Fast_Dinic() {       long long ans = 0;       for (int j = 29; j >= 0; j--) {         for (int i = 0; i < n; i++) {           for (int k = 0; k < (int)g[i].size(); k++) {             if (g[i][k].c & (1 << j)) g[i][k].c2 += (1 << j);           }         }         ans += Dinic();       }       return ans;     }   };   bool is_prime(int x) {     for (int i = 2; i * i <= x; i++) {       if (x % i == 0) return false;     }     return true;   }   vector<int> usd;   vector<vector<int>> gr;   void dfs_ans(int v, vector<int> &comp) {     comp.emplace_back(v);     usd[v] = 1;     for (auto &to : gr[v]) {       if (!usd[to]) dfs_ans(to, comp);     }   }   int main() {     ios_base::sync_with_stdio(0);     cin.tie(0);     cout.tie(0);     int n;     cin >> n;     vector<int> a(n);     int cnt0 = 0, cnt1 = 0;     for (auto &c : a) {       cin >> c;       if (c % 2 == 0)         cnt0++;       else         cnt1++;     }     if (cnt0 != cnt1) {       cout <<  Impossible ;       return 0;     }     Graph g(n + 2, n, n + 1);     for (int i = 0; i < n; i++) {       if (a[i] % 2 == 1) {         g.add_dir(n, i, 2, -1);       } else {         g.add_dir(i, n + 1, 2, i);       }     }     for (int i = 0; i < n; i++) {       for (int j = 0; j < n; j++) {         if (a[i] % 2 == 1 && a[j] % 2 == 0) {           if (is_prime(a[i] + a[j])) {             g.add_dir(i, j, 1, -1);           }         }       }     }     int x = g.Edmonds_Karp();     if (x != 2 * cnt0) {       cout <<  Impossible ;       return 0;     }     gr.resize(n);     vector<vector<int>> ans;     for (int i = 0; i < n; i++) {       for (auto &e : g.g[i]) {         if (e.f == 1) {           gr[i].emplace_back(e.to);           gr[e.to].emplace_back(i);         }       }     }     usd.assign(n, 0);     for (int i = 0; i < n; i++) {       if (!usd[i]) {         ans.emplace_back(vector<int>());         dfs_ans(i, ans.back());       }     }     cout << ans.size() <<   n ;     for (auto &c : ans) {       cout << c.size() <<    ;       for (auto &l : c) {         cout << l + 1 <<    ;       }       cout <<   n ;     }     return 0;   }   