class CharIterator
  def initialize(str)
    @str = str
    @idx = 0
  end
 
  def next
    begin
      @str[@idx]
    ensure
      @idx += 1
    end
  end
 
  def has_next?
    @idx < @str.length
  end
end
 
class CharSet
  def initialize(str)
    @str = str
  end
 
  def iterator
    CharIterator.new(@str)
  end
end
 
#### main ####
 
char_set = CharSet.new('abcdefghijklmn')
iterator = char_set.iterator
 
while iterator.has_next?
  puts iterator.next
end

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