Problem:
Problem Statement

We found a dictionary of the Ancient Civilization Mayo (ACM) during excavation of the ruins.
After analysis of the dictionary, we revealed they used a language that had not more than 26 letters.
So one of us mapped each letter to a different English alphabet and typed all the words in the dictionary into a computer.

How the words are ordered in the dictionary, especially whether they are ordered lexicographically, is an interesting topic to many people.
As a good programmer, you are requested to write a program to judge whether we can consider the words to be sorted in a lexicographical order.

Note:
In a lexicographical order, a word always precedes other words it is a prefix of.
For example, ab
precedes abc,
abde, and so on.

Input

The input consists of multiple datasets. Each dataset is formatted as follows:

n
string_1
...
string_n

Each dataset consists of n+1 lines.
The first line of each dataset contains an integer that indicates n (1 \leq n \leq 500).
The i-th line of the following n lines contains string_i, which consists of up to 10 English lowercase letters.

The end of the input is 0, and this should not be processed.

Output

Print either yes or
no in a line for each dataset, in the order of the input.
If all words in the dataset can be considered to be ordered lexicographically, print yes.
Otherwise, print no.

Sample Input

4
cba
cab
b
a
3
bca
ab
a
5
abc
acb
b
c
c
5
abc
acb
c
b
b
0

Output for the Sample Input

yes
no
yes
no

Solution in Ruby (judge verdict: Wrong Answer)

while (inp = gets) && inp != '0'
  ss = []
  n = inp.to_i
  n.times{ss << gets.chomp}
  g = Array.new(26){Array.new(26,false)}
  ng = false
  n.times{|i|
    i.times{|j|
      if [ss[i].size,ss[j].size].min.times{|k|
          if ss[i][k] != ss[j][k]
            g[ss[j][k].ord-'a'.ord][ss[i][k].ord-'a'.ord] = true
            break
          end
        }
        ng = true if ss[i] != ss[j]
      end
    }
  }
  26.times{|k|
    26.times{|i|
      26.times{|j|
        g[i][j] |= g[i][k]&&g[k][j]
        ng |= i==j && g[i][j]
      }
    }
  }
  puts ng ? 'no' : 'yes'
end