#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef unsigned long long int ull;

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
ll myRand(ll B) {
    return (ull)rng() % B;
}
inline double time() {
    return static_cast<long double>(chrono::duration_cast<chrono::nanoseconds>(chrono::steady_clock::now().time_since_epoch()).count()) * 1e-9;
}

void slv() {
    int n,m; cin >> n >> m;
    vector<vector<int>> g(n), r(m);

    for (int i = 0; i < n-1; ++i) {
        int x; cin >> x; x -= 1;
        g[x].push_back(i);
    }
    for (int i = 0; i < m-1; ++i) {
        int x; cin >> x; x -= 1;
        r[x].push_back(i);
    }
    int k = 0;
    for (int i = 0; i < n; ++i) {
        if (g[i].size() == 0) k += 1;
    }
    vector<int> col(k, -1);
    vector<vector<int>> v1(k), v2(k);
    vector<vector<int>> rv1(n), rv2(m);
    vector<int> rem1(n), rem2(m);
    vector<int> val1(n), val2(m);
    {
        auto dfs = [&](auto dfs, int s) -> vector<int> {
            if (g[s].size() == 0) {
                return vector<int>{s};
            }
            else if (g[s].size() == 1) {
                return dfs(dfs, g[s][0]);
            }
            else {
                vector<int> v;
                for (int t : g[s]) {
                    auto r = dfs(dfs, t);
                    rem1[s] += r.size();
                    if (v.size() < r.size()) swap(v, r);
                    for (int &vv : r) {
                        v.push_back(vv);
                    }
                }
                for (int t : v) {
                    v1[t].push_back(s);
                }
                rv1[s] = v;
                return v;
            }
        };
        dfs(dfs, n-1);
    }
    {
        auto dfs = [&](auto dfs, int s) -> vector<int> {
            if (r[s].size() == 0) {
                return vector<int>{s};
            }
            else if (r[s].size() == 1) {
                return dfs(dfs, r[s][0]);
            }
            else {
                vector<int> v;
                for (int t : r[s]) {
                    auto rr = dfs(dfs, t);
                    rem2[s] += rr.size();
                    if (v.size() < rr.size()) swap(v, rr);
                    for (int &vv : rr) {
                        v.push_back(vv);
                    }
                }
                for (int t : v) {
                    v2[t].push_back(s);
                }
                rv2[s] = v;
                return v;
            }
        };
        dfs(dfs, m-1);
    }

    for (int i = 0; i < k; ++i) {
        if (col[i] != -1) continue;
        queue<int> q;
        auto psh = [&](int s) -> void {
            for (int &t : v1[s]) {
                rem1[t] -= 1;
                val1[t] += (col[s] == 0 ? 1 : -1);
            }
            for (int &t : v2[s]) {
                rem2[t] -= 1;
                val2[t] += (col[s] == 0 ? 1 : -1);
            }
            q.push(s);
        };
        col[i] = 0; psh(i); q.push(i);
        while (q.size()) {
            int s = q.front(); q.pop();
            for (int &t : v1[s]) {
                int v = abs(val1[t]);
                int c = rem1[t];
                // 確定
                if (v == c or v == c+1) {
                    if (c == 0) continue;
                    for (int &tt : rv1[t]) {
                        if (col[tt] == -1) {
                            if (val1[t] > 0) {
                                col[tt] = 1;
                            }
                            else {
                                col[tt] = 0;
                            }
                            psh(tt);
                        }
                    }
                }
                // NG
                if (v > c+1) {
                    cout << "IMPOSSIBLE" << "\n";
                    return;
                }
            }
            for (int &t : v2[s]) {
                int v = abs(val2[t]);
                int c = rem2[t];
                // 確定
                if (v == c or v == c+1) {
                    if (c == 0) continue;
                    for (int &tt : rv2[t]) {
                        if (col[tt] == -1) {
                            if (val2[t] > 0) {
                                col[tt] = 1;
                            }
                            else {
                                col[tt] = 0;
                            }
                            psh(tt);
                        }
                    }
                }
                // NG
                if (v > c+1) {
                    cout << "IMPOSSIBLE" << "\n";
                    return;
                }
            }
        }
    }

    for (int i = 0; i < k; ++i) {
        if (col[i] == 0) cout << "B";
        else if (col[i] == 1) cout << "R";
        else cout << "?";
    }
    cout << "\n";
}

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int q; cin >> q;
    while (q--) {
        slv();
    }
}