#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <iomanip>
#include <utility>
#include <tuple>
#include <functional>
#include <bitset>
#include <cassert>
#include <complex>
#include <stdio.h>
#include <time.h>
#include <numeric>
#include <random>
#include <unordered_map>
#include <unordered_set>
#define all(a) a.begin(),a.end()
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define pb push_back
#define debug(x) cerr << #x << ':' << (x) << '\n'
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> P;
typedef complex<ld> com;
constexpr int inf = 1000000010;
constexpr ll INF = 1000000000000000010;
constexpr ld eps = 1e-12;
constexpr ld pi = 3.141592653589793238;
template<class T, class U> inline bool chmax(T &a, const U &b) { if (a < b) { a = b; return true; } return false; }
template<class T, class U> inline bool chmin(T &a, const U &b) { if (a > b) { a = b; return true; } return false; }

constexpr ll mod = 1000000007;
vector<ll> fac, inv, facinv;

void modcalc(int modsize) {
	fac.resize(modsize); inv.resize(modsize); facinv.resize(modsize);
	fac[0] = 1; fac[1] = 1; inv[1] = 1;
	facinv[0] = 1; facinv[1] = 1;
	for (ll i = 2; i < modsize; i++) {
		fac[i] = fac[i - 1] * i % mod;
		inv[i] = mod - inv[mod % i] * (mod / i) % mod;
		facinv[i] = facinv[i - 1] * inv[i] % mod;
	}
}

ll modinv(ll a) {
	a %= mod;
	if (a == 0) abort();
	if (a < (ll)inv.size()) return inv[a];
	ll b = mod, u = 1, v = 0;
	while (b) {
		ll t = a / b;
		a -= t * b; swap(a, b);
		u -= t * v; swap(u, v);
	}
	u %= mod;
	if (u < 0) u += mod;
	return u;
}

ll modpow(ll a, ll b, ll m = mod) {
	ll ans = 1;
	a %= m;
	while (b) {
		if (b & 1) ans = ans * a % m;
		a = a * a % m;
		b >>= 1;
	}
	return ans;
}

ll modcomb(ll n, ll k) {
	if (n < 0 || k < 0 || n < k) return 0;
	return fac[n] * facinv[k] % mod * facinv[n - k] % mod;
}

ll modperm(ll n, ll k) {
	if (n < 0 || k < 0 || n < k) return 0;
	return fac[n] * facinv[n - k] % mod;
}

ll modhom(ll n, ll k) {
	if (n < 0 || k < 0 || n == 0 && k > 0) return 0;
	if (n == 0 && k == 0) return 1;
	return fac[n + k - 1] * facinv[k] % mod * facinv[n - 1] % mod;
}

const int N = 200010;
vector<vector<int>> graph(N, vector<int>());

void dfs(int n, vector<set<int>> &st, map<ll, ll> &memo) {
	ll pro = 1;
	ll inv = 0;
	int cnt = 0;
	for (int i : st[n]) {
		dfs(i, st, memo);
		pro *= mod + 1 - memo[i]; pro %= mod;
		inv += memo[i] * modinv(mod + 1 - memo[i]) % mod;
		cnt++;
	}
	if (!cnt) return;
	inv %= mod; if (inv < 0) inv += mod;
	memo[n] = pro * inv % mod;
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout << fixed << setprecision(20);

	int n;
	cin >> n;
	for (int i = 1; i <= n; i++) {
		int v;
		cin >> v;
		graph[v].pb(i);
	}
	vector<set<int>> st(n + 1);
	queue<int> que;
	que.push(0);
	vector<int> par(n + 1, -1);
	ll ans = (mod + 1) / 2;
	rep(i, n + 1) {
		map<ll, ll> mp;
		queue<int> que2;
		while (!que.empty()) {
			int v = que.front(); que.pop();
			if (graph[v].size() >= 2) {
				for (int nxt : graph[v]) {
					st[v].insert(nxt);
					par[nxt] = v;
					que2.push(nxt);
					mp[nxt] = (mod + 1) / 2;
				}
			}
			else if (graph[v].size() == 1) {
				int nxt = graph[v][0];
				int pr = par[v];
				if (pr != -1) {
					st[pr].erase(v);
					st[pr].insert(nxt);
					par[nxt] = pr;
				}
				else {
					st[v].insert(nxt);
					par[nxt] = v;
				}
				que2.push(nxt);
				mp[nxt] = (mod + 1) / 2;
			}
			else {
				int now = v;
				while (par[now] != -1 && st[now].size() == 0) {
					int pr = par[now];
					st[pr].erase(now);
					now = pr;
				}
			}
		}
		que = que2;
		dfs(0, st, mp);
		ans += mp[0];
	}
	ans %= mod;
	ans = ans * modpow(2, n + 1) % mod;
	cout << ans << '\n';
}

Measured memory_bytes: 42516