func_code_string: #include <bits/stdc++.h>   using namespace std;   const int mod = 1e9 + 7;   int i, j, k, n, m, s, t, ans1, ans2, ans;   int dp[1005][1 << 12];   struct Tree {     int head[1005];     struct edge {       int vet, next;     } E[2005];     int son[1005];     int num[1005];     int a[1005][1005];     int n, tot;     Tree() {       memset(head, -1, sizeof(head));       tot = -1;     }     void add(int u, int v) {       E[++tot] = (edge){v, head[u]};       head[u] = tot;     }     void read() {       scanf( %d , &n);       for (int i = 1; i < n; i++) {         int x, y;         scanf( %d%d , &x, &y);         add(x, y);         add(y, x);       }     }     void dfs(int u, int dad) {       son[u] = 0;       for (int e = head[u]; e != -1; e = E[e].next)         if (e != dad) {           son[u] |= 1 << (E[e].vet - 1);           dfs(E[e].vet, e ^ 1);         }     }     void getsize(int u, int dad) {       num[u] = 0;       for (int e = head[u]; e != -1; e = E[e].next)         if (e != dad) {           a[u][++num[u]] = E[e].vet;           getsize(E[e].vet, e ^ 1);         }     }   } S, T;   int powmod(const int &x, int y) {     if (y == 0) return 1;     int ret = powmod(x, y >> 1);     if (y & 1)       ret = (long long)ret * ret % mod * x % mod;     else       ret = (long long)ret * ret % mod;     return ret;   }   int dfs(int u, int num, int opt) {     if (num == 0) return opt == 0;     int x = S.a[u][num];     if (dp[x][opt] != -1) return dp[x][opt];     dp[x][opt] = dfs(u, num - 1, opt);     for (int i = 0; i < T.n; i++)       if ((1 << i) & opt) {         dp[x][opt] += (long long)dfs(u, num - 1, opt ^ (1 << i)) *                       dfs(x, S.num[x], T.son[i + 1]) % mod;         if (dp[x][opt] >= mod) dp[x][opt] -= mod;       }     return dp[x][opt];   }   int main() {     S.read();     T.read();     S.getsize(1, -1);     for (int j = 1; j <= T.n; j++) {       memset(dp, -1, sizeof(dp));       T.dfs(j, -1);       for (int i = 1; i <= S.n; i++) {         ans1 += dfs(i, S.num[i], T.son[j]);         if (ans1 >= mod) ans1 -= mod;       }     }     S = T;     S.getsize(1, -1);     for (int j = 1; j <= T.n; j++) {       memset(dp, -1, sizeof(dp));       T.dfs(j, -1);       ans2 += dfs(1, S.num[1], T.son[j]);       if (ans2 >= mod) ans2 -= mod;     }     ans = (long long)ans1 * powmod(ans2, mod - 2) % mod;     printf( %d n , ans);     return 0;   }   