#include <bits/stdc++.h>
#define N ('z'-'a')
using namespace std;
int n,ans,mp[N+1];
string str[501];

void mk_mp(int pos,int i,int j){
  while(i<=j&&pos>=str[i].size())i++;
  if(i>=j)return;
  int ni=i,ch=str[i][pos]-'a';

  while(i<j){
    int a=str[++i][pos]-'a',f=ch==a;
    if((f&&!ni<0)||(f&&str[i-1].size()>str[i].size()&&!str[i][pos+1]))ans=0;

    if(!f){
      mk_mp(pos+1,ni,i-1);
      mk_mp(pos,i,j);
      mp[ch]|=(1<<a|mp[a]);
      return ;
    }
  }
  if(ni>=0) mk_mp(pos+1,ni,j);
}

int main(){
  while(cin>>n,n){
    for(int i=0;i<n;i++)cin>>str[i];
    ans=1;
    mk_mp(0,0,n-1);
    for(int i=0;i<=N;mp[i]=0,i++) ans*=!(mp[i]&1<<i);
    cout <<(ans? "yes":"no")<<endl;
  }
  return 0;
}