def to_ru!
      self.each_with_index do |x, row, col|
        if x.nil? || x.value.nil?
          self[row, col] = nil
        else
          self[row, col] = x.to_ru
        end
      end
    end
def to_ru!
      self.each_with_index do |x, row, col|
        if x.nil? || x.value.nil?
          self[row, col] = nil
        else
          self[row, col] = x.to_ru
        end
      end
    end
Invokes the #Cell::to_ru method on each element of self, replacing each element of the Sheet with the value returned.