def parse(node)
      @borders = true if node.name == 'borderBox'
      @element = MathRun.new(parent: self).parse(node)
      self
    end
def parse(node)
      @borders = true if node.name == 'borderBox'
      @element = MathRun.new(parent: self).parse(node)
      self
    end
Parse Box object
 @param node [Nokogiri::XML:Element] node to parse
 @return [Box] result of parsing