class ReverseCharIterator
  def initialize(str)
    @str = str
    @idx = str.length - 1
  end
 
  def next
    begin
      @str[@idx]
    ensure
      @idx -= 1
    end
  end
 
  def has_next?
    @idx >= 0
  end
end
 
class ReverseCharSet
  def initialize(str)
    @str = str
  end
 
  def iterator
    ReverseCharIterator.new(@str)
  end
end
 
#### main ####
 
char_set = ReverseCharSet.new('abcdefghijklmn')
iterator = char_set.iterator
 
while iterator.has_next?
  puts iterator.next
end

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