#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
using namespace std;

int main(){
    int N;
    while(cin>>N,N){
        vector<string> S(N);
        for(auto &s:S) cin>>s;
        int G[26][26];
        fill(G[0],G[26],0);
        bool flag=true;
        for(int i=0;i<N;i++){
            for(int j=i+1;j<N;j++){
                string s1=S[i], s2=S[j];
                if(s1==s2) continue;
                int ite=0;
                while(ite<min(s1.size(),s2.size())&&s1[ite]==s2[ite]) ite++;
                if(ite==s1.size()) continue;
                if(ite==s2.size()){
                    flag=false;
                    continue;
                }
                G[s1[ite]-'a'][s2[ite]-'a']=1;
            }
        }
        for(int k=0;k<26;k++){
            for(int i=0;i<26;i++){
                for(int j=0;j<26;j++){
                    G[i][j]|=G[i][k]&G[k][j];
                }
            }
        }
        for(int i=0;i<26;i++){
            if(G[i][i]) flag=false;
        }
        cout<<(flag?"yes":"no")<<endl;
    }

    return 0;
}
