output_description: Assume that there are K strings of length N that are in normal form: w_1,
  \ldots, w_K in lexicographical order. Output should be in the following
  format:
  
      
      
      w_1
      :
      w_K
      
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
attempt: import itertools
  
  bells = [[], [[0]]] + [None for i in range(9)]
  bell_n = [1, 1, 2, 5, 15, 52, 203, 877, 4140, 21147, 115975]
  
  
  def search(n):
      ret = []
      ret.append([0 for _ in range(n)])
      for k in range(1, n):
          for bi in bells[k]:
              for ids in itertools.combinations(range(1, n), k):
                  st = [0 for _ in range(n)]
                  for i, id in enumerate(ids):
                      st[id] = bi[i] + 1
                  ret.append(st)
      return ret
  
  
  for i in range(2, 10):
      bells[i] = search(i)
      assert bell_n[i] == len(bells[i])
  
  N = int(input())
  table = "abcdefghij"
  outs = []
  for bi in bells[N]:
      out = ""
      for b in bi:
          out += table[b]
      outs.append(out)
  
  print("\n".join(sorted(outs)))
  
problem_description: Statement
  
  In this problem, we only consider strings consisting of lowercase English
  letters.
  
  Strings s and t are said to be **isomorphic** when the following conditions
  are satisfied:
  
    * |s| = |t| holds.
    * For every pair i, j, one of the following holds:
      * s_i = s_j and t_i = t_j.
      * s_i \neq s_j and t_i \neq t_j.
  
  For example, `abcac` and `zyxzx` are isomorphic, while `abcac` and `ppppp` are
  not.
  
  A string s is said to be in **normal form** when the following condition is
  satisfied:
  
    * For every string t that is isomorphic to s, s \leq t holds. Here \leq denotes lexicographic comparison.
  
  For example, `abcac` is in normal form, but `zyxzx` is not since it is
  isomorphic to `abcac`, which is lexicographically smaller than `zyxzx`.
  
  You are given an integer N. Print all strings of length N that are in normal
  form, in lexicographically ascending order.
samples: [{"input": "1", "output": "a\n    \n\n* * *"}, {"input": "2", "output": "aa\n    ab"}]