#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
#define ff first
#define ss second
#define pb push_back
typedef long long ll;
using namespace std;
typedef pair<int,int> pii;

const int mod=998244353;
inline int add(int x,int y){int ret=x+y;if(ret>=mod)ret-=mod;return ret;}
inline int sub(int x,int y){int ret=x-y;if(ret<0)ret+=mod;return ret;}
inline int mul(int x,int y){return ((ll)x*y)%mod;}
inline int step(int base,int pw){int ret=1;while(pw){if(pw&1)ret=mul(ret,base);base=mul(base,base);pw>>=1;}return ret;}
inline int invv(int x){return step(x,mod-2);}


const int maxn=2e5+10;

vector<int>rez_edges;
vector<int>g[maxn],vect[maxn];
struct edge{
    int u,v,flow,cap;
};
vector<edge>e;
int incnt[maxn],outcnt[maxn],pos[maxn],sz[maxn],n,m;
int sink,source;
int level[maxn],start[maxn];

void add_edge(int x,int y,int cap){

    if(cap==0)return;

    vect[x].pb(e.size());
    e.pb({x,y,0,cap});

    vect[y].pb(e.size());
    e.pb({y,x,cap,cap});

   /// printf("%d %d | %d\n",x,y,cap);

}


void go(int x){

    int child=0;
    sz[x]=0;
    incnt[x]=0;
    outcnt[x]=0;
    for(int i=0;i<g[x].size();i++){
        int id=g[x][i];
        go(id);
        child++;
        sz[x]+=sz[id];

        if(x<=n){
            if(sz[id]%2)add_edge(x,id,1);
            outcnt[x]+=sz[id]/2;
            incnt[id]+=sz[id]/2;
        }
        else{
            if(sz[id]%2)add_edge(id,x,1);
            outcnt[id]+=sz[id]/2;
            incnt[x]+=sz[id]/2;
        }
    }

    if(child==0){
        if(x<=n)rez_edges.pb(e.size());
        if(x<=n)add_edge(x,x+n,1);
        sz[x]=1;
    }

}

int q[maxn];
int bfs(){

        memset(level,0,sizeof(level));
    level[source]=1;
    int tr=0,l=0;
    q[tr++]=source;
    while(l!=tr){
        int x=q[l++];
        for(int i=0;i<vect[x].size();i++){
            int id=vect[x][i];
            if(e[id].flow>=e[id].cap || level[e[id].v])continue;
            level[e[id].v]=level[x]+1;
            q[tr++]=e[id].v;
        }
    }
    return level[sink];
}
int send_flow(int x,int flow){

    if(x==sink)return flow;

    for(;start[x]<vect[x].size();start[x]++){
        int id=vect[x][start[x]];

        if(e[id].flow<e[id].cap && level[x]+1==level[e[id].v]){

            int tmpf=send_flow(e[id].v,min(flow,e[id].cap-e[id].flow));

            if(tmpf){
                e[id].flow+=tmpf;
                e[id^1].flow-=tmpf;
                return tmpf;
            }

        }

    }

    return 0;
}
ll calc_flow(){

    ll ret=0;
    while(bfs()){
        memset(start,0,sizeof(start));
        ll pom;
        while((pom=send_flow(source,1e9)))ret+=pom;
    }
    return ret;
}

int main(){

    ///freopen("test.txt","r",stdin);


    int t;
    scanf("%d",&t);
    while(t--){

        scanf("%d %d",&n,&m);
        e.clear();
        rez_edges.clear();
        n++;
        m++;
        for(int i=0;i<=n+m+1;i++){
            g[i].clear();
            vect[i].clear();
        }
        for(int i=1;i<n-1;i++){
            int a;
            scanf("%d",&a);
            g[a].pb(i);
        }
        for(int i=1;i<m-1;i++){
            int a;
            scanf("%d",&a);
            g[a+n].pb(i+n);
        }
        g[n].pb(n-1);
        g[n+m].pb(n+m-1);
        source=0;
        sink=n+m+1;

        go(n);
        go(m+n);

        ll sum=0;
        for(int i=1;i<=n+m;i++){
            int pom=min(incnt[i],outcnt[i]);
            incnt[i]-=pom;
            outcnt[i]-=pom;
            add_edge(source,i,incnt[i]);
            add_edge(i,sink,outcnt[i]);
            sum+=incnt[i];
        }
        add_edge(m+n,n,1e9);

        ///printf("%lld SUM\n",sum);

        ll pom=calc_flow();
        if(pom!=sum){
            printf("IMPOSSIBLE\n");
            continue;
        }
        for(int i=0;i<rez_edges.size();i++){
            int id=rez_edges[i];
            int v=e[id].u;
            pos[v]=e[id].flow;
        }

        for(int i=1;i<=rez_edges.size();i++){
            if(pos[i]==0)printf("R");
            else printf("B");
        }
        printf("\n");

    }

    return 0;
}