//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <climits>
#include <queue>

using namespace std;

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);

int main(){
  cin.tie(0);
  ios_base::sync_with_stdio(false);

  int N;
  while(cin>>N,N){
	VS vs(N);
	REP(i,N) cin >> vs[i];
	int dist[32][32];
	fill((int*)dist, (int*)dist+32*32, 100);

	bool ans = true;
	for(int i=0;i<N;++i)
	  for(int j=i+1;j<N;++j){
		int k;
		for(k=0;k<min(SZ(vs[i]),SZ(vs[j]));++k){
		  if(vs[i][k] != vs[j][k]){
			dist[vs[i][k]-'a'][vs[j][k]-'a'] = 0;
			break;
		  }
		}
		if(k == SZ(vs[j]) && SZ(vs[i]) > SZ(vs[j])) ans = false;
	  }

	REP(k,32) REP(i,32) REP(j,32)
	  dist[i][j] = min(dist[i][j], dist[i][k] + dist[k][j]);

	REP(i,32) if(!dist[i][i]) ans = false;
	
	cout << (ans? "yes": "no") << endl;
  }
  
  return 0;
}