#include <bits/stdc++.h>
using namespace std;
#define FOR(i,k,n) for(int i = (k); i < (n); i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(a) a.begin(), a.end()
#define MS(m,v) memset(m,v,sizeof(m))
#define D10 fixed<<setprecision(10)
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<string> vs;
typedef pair<ll, ll> pii;
const int MOD = 1000000007;
const int INF = MOD + 1;
const ld EPS = 1e-12;
template<class T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template<class T> T &chmax(T &a, const T &b) { return a = max(a, b); }

/*--------------------template--------------------*/

int main()
{
	int n;
	while (cin >> n, n)
	{
		vector<vi> g(27);
		vs v;
		REP(i, n)
		{
			string s; cin >> s;
			while (s.size() < 10) s += 'z' + 1;
			v.push_back(s);
		}
		REP(i, n)REP(j, i)
		{
			string u = v[i], d = v[j];
			int t = 0;
			while (u[t] == d[t]&&t<10) t++;
			if (t == 10) continue;
			int un = u[t] - 'a', dn = d[t] - 'a';
			if (un != dn) g[un].push_back(dn);
		}
		bool f = true;
		if (g[26].size() > 0) f = false;
		REP(i, 26)
		{
			sort(ALL(g[i]));
			g[i].erase(unique(ALL(g[i])), g[i].end());
		}
		vector<bool> vis(26);
		REP(i, 26)
		{
			stack<pii> st;
			st.push(pii(i, 0));
			while (st.size())
			{
				int tmp = st.top().first, cnt = st.top().second;
				st.pop();
				if (cnt>30)
				{
					f = false;
					goto end;
				}
				REP(j, g[tmp].size())
				{
					st.push(pii(g[tmp][j], cnt+1));
				}
			}
		}
	end:;
		puts(f ? "yes" : "no");
	}
	return 0;
}