#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
const int inf = 1e9;
const int AtoZ = 26;

bool check(int n, int b, int e, vector<string> &str, vector<vector<int> > &ord){
	char now=str[b][n];
	int s=b;
	for(int i=b; i<=e; i++){
		if(str[i][n] != now){
			if(str[i][n]=='0') return false;
			if(now!='0'){
				ord[now-'a'][str[i][n]-'a'] = 0;
				if(!check(n+1, s, i-1, str, ord)) return false;
			}
			s = i;
			now = str[i][n];
		}
	}
	if(now!='0'){
		if(!check(n+1, s, e, str, ord)) return false;
	}
	return true;
}

int main(){
	while(1){
		int n;
		cin >> n;
		if(n==0) break;
		
		vector<string> str(n);
		vector<vector<int> > ord(AtoZ, vector<int>(AtoZ, inf));
		for(int i=0; i<n; i++){
			cin >> str[i];
			str[i] += "0";
		}
		if(!check(0, 0, n-1, str, ord)){
			cout << "no" << endl;
			continue;
		}
		
		for(int k=0; k<AtoZ; k++){
			for(int i=0; i<AtoZ; i++){
				for(int j=0; j<AtoZ; j++){
					ord[i][j] = min(ord[i][j], ord[i][k]+ord[k][j]);
				}
			}
		}
		bool flag = true;
		for(int i=0; i<AtoZ; i++){
			for(int j=i+1; j<AtoZ; j++){
				if(ord[i][j]==0 && ord[j][i]==0){
					flag = false;
					break;
				}
			}
		}
		if(flag){
			cout << "yes" << endl;
		}else{
			cout << "no" << endl;
		}
	}
    return 0;
}