// 文字に適切な順序を定めて、入力が辞書順になるようにできるか
// 適当に辺を貼って閉路見つける

#include <bits/stdc++.h>
using namespace std;

int n;
string s[500];
bool f[500][500];
bool g[27][27];

int main()
{
	while (cin >> n, n){
		memset(f, 0, sizeof(f));
		memset(g, 0, sizeof(g));
		for (int i = 1; i < 27; i++){
			g[0][i] = true;
		}

		for (int i = 0; i < n; i++){
			cin >> s[i];
			s[i] += 'a' - 1;
		}
		
		for (int i = 0; i <= 10; i++){
			for (int j = 0; j < n; j++){
				for (int k = j + 1; k < n; k++){
					if (f[j][k]) continue;
					if (i >= min(s[j].size(), s[k].size())) continue;
					if (s[j][i] != s[k][i]){
						g[s[j][i] - 'a' + 1][s[k][i] - 'a' + 1] = true;
						f[j][k] = true;
					}
				}
			}
		}

		for (int k = 0; k < 27; k++){
			for (int i = 0; i < 27; i++){
				for (int j = 0; j < 27; j++){
					g[i][j] |= g[i][k] && g[k][j];
				}
			}
		}
		
		bool res = true;
		for (int i = 0; i < 27; i++){
			if (g[i][i]) res = false;
		}
		
		cout << (res ? "yes" : "no") << endl;
	}
}