# include<iostream>

# include<cstdio>

# include<vector>

# include<queue>

# include<cstring>

# include<algorithm>

using namespace std;

# define LL long long

# define REP(i,s,n) for(int i=s;i<n;++i)

# define CLS(a,b) memset(a,b,sizeof(a))

# define CLL(a,b,n) fill(a,a+n,b)

const int N=105;

vector<int>G[N<<1];

int n,m,a[5],b[5],must[N+N],mark[N+N],flag[N],s[N+N],cnt;

void add(int x,int u,int y,int v)

{

    x=x*2+u;

    y=y*2+v;

    G[x^1].push_back(y);

    G[y^1].push_back(x);

}

void dfs1(int u)

{

    must[u]=1;

    REP(i,0,G[u].size()) if(!must[G[u][i]]) dfs1(G[u][i]);

}

void clear()

{

    CLS(mark,0);

    REP(i,0,n+n) mark[i]=must[i];

}

bool dfs(int x)

{

    if(mark[x^1]) return false;

    if(mark[x]) return true;

    mark[x]=1;

    s[cnt++]=x;

    REP(i,0,G[x].size()) if(!dfs(G[x][i])) return false;

    return true;

}

bool solve()

{

    for(int i=0;i<n+n;i+=2){

        if(mark[i]&&mark[i+1]) return false;

        if(!mark[i]&&!mark[i+1]){

            cnt=0;

            if(!dfs(i)){

                while(cnt>0) mark[s[--cnt]]=0;

                if(!dfs(i+1)) return false;

            }

        }

    }

    return true;

}

void init()

{

    REP(i,0,n*2) G[i].clear();

    CLS(must,0);

    CLS(mark,0);

}

int main()

{

    int k,cas=0;

    char c;

    while(scanf("%d%d",&n,&m)&&(n+m))

    {

        init();

        CLS(flag,0);

        while(m--)

        {

            scanf("%d",&k);

            REP(i,0,k){

                scanf("%d %c",&a[i],&c);

                --a[i];

                if(c=='y') b[i]=1;

                else b[i]=0;

            }

            if(k<=2){

                REP(i,0,k) must[a[i]*2+b[i]]=1;

            }else

                REP(i,0,k) REP(j,i+1,k) add(a[i],b[i],a[j],b[j]);

        }

        REP(i,0,n+n) if(must[i]) dfs1(i);

        printf("Case %d: ",++cas);

        REP(i,0,n){

            if(must[i*2]||must[i*2+1]) continue;

            clear();

            mark[i*2]=1;

            bool yy1=solve();

            clear();

            mark[i*2+1]=1;

            bool yy2=solve();

            if(yy1&&yy2) flag[i]=1;

        }

        clear();

        if(!solve()){

            printf("impossible\n");

            continue;

        }

        REP(i,0,n){

            if(flag[i]) printf("?");

            else if(mark[i*2]) printf("n");

            else printf("y");

        }

        printf("\n");

    }

    return 0;

}