def mix_r!(i, j, c = nil)
      @left.mix_r!(i, j, c)
      @body.mix_r!(i, j, c)
      self
    end
def mix_r!(i, j, c = nil)
      @left.mix_r!(i, j, c)
      @body.mix_r!(i, j, c)
      self
    end
def divide_c(j, c)
    dup.divide_c!(j, c)
  end