#include <bits/stdc++.h>
using namespace std;
static const double EPS = 1e-8;
const static int tx[] = {0, 1, 0, -1};
const static int ty[] = {-1, 0, 1, 0};
int main() {
  int N;
  while (~scanf("%d", &N)) {
    if (N == 0) break;
    vector<string> dict;
    int prior[256][256];
    memset(prior, -1, sizeof(prior));
    bool isok = true;
    bool res = true;
    for (int current_idx = 0; current_idx < N; current_idx++) {
      string current;
      cin >> current;
      for (int prev_idx = 0; prev_idx < dict.size(); prev_idx++) {
        for (int char_idx = 0;
             char_idx < current.size() && char_idx < dict[prev_idx].size();
             char_idx++) {
          if (current[char_idx] == dict[prev_idx][char_idx]) continue;
          if (prior[dict[prev_idx][char_idx]][current[char_idx]] == -1) {
            prior[dict[prev_idx][char_idx]][current[char_idx]] = 1;
            prior[current[char_idx]][dict[prev_idx][char_idx]] = 0;
          }
        }
        bool isok = true;
        bool is_first = true;
        for (int char_idx = 0;
             char_idx < current.size() && char_idx < dict[prev_idx].size();
             char_idx++) {
          if (current[char_idx] == dict[prev_idx][char_idx]) continue;
          if (is_first &&
              prior[current[char_idx]][dict[prev_idx][char_idx]] == 1) {
            isok = false;
            break;
          }
          is_first = false;
        }
        if (isok) {
          if (current.size() < dict[prev_idx].size() &&
              dict[prev_idx].substr(0, current.size()) == current) {
            isok = false;
          }
        }
        res &= isok;
      }
      dict.push_back(current);
    }
  found:;
    printf("%s\n", res ? "yes" : "no");
  }
}
