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// SRM 683, d2-hard, by Errichto
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int nax = 1e6 + 5;
int a[nax];
vector<int> w[nax];
const int mod = 1e9 + 7;
int answer;
 
int mul(int a, int b) { return (ll) a * b % mod; }
 
pair<int,int> dfs(int a, int par) {
	int pref = 1, ways = 1;
	for(int b : w[a]) if(b != par) {
		pair<int,int> tmp = dfs(b, a);
		pref = add(mul(pref, tmp.second), mul(ways, tmp.first));
		ways = mul(ways, tmp.second);
	}
	answer = add(answer, pref);
	return make_pair(pref, ways + 1);
}
 
class SubtreesCounting {
	public : int sumOfSizes(int n, int a0, int b, int c, int m) {
		a[0] = a0;
		for(int i = 1; i <= n - 2; ++i)
			a[i] = ((ll) b * a[i-1] + c) % m;
		for(int i = 1; i <= n - 1; ++i) {
			int j = a[i-1] % i;
			// edge between i and j
			w[i].push_back(j);
			w[j].push_back(i);
		}
		dfs(0, -1);
		return answer;
	} 
};

prog.cpp: In function 'std::pair<int, int> dfs(int, int)':
prog.cpp:17:57: error: 'add' was not declared in this scope
   pref = add(mul(pref, tmp.second), mul(ways, tmp.first));
                                                         ^
prog.cpp:20:27: error: 'add' was not declared in this scope
  answer = add(answer, pref);
                           ^

Standard output is empty