_toHTML (options) {
      const items = this.items.map(function (node) {
        return node.toHTML(options)
      })
      return '<span class="math-parenthesis math-square-parenthesis">[</span>' +
        items.join('<span class="math-separator">,</span>') +
        '<span class="math-parenthesis math-square-parenthesis">]</span>'
    }
Get HTML representation
@param {Object} options
@return {string} str
@override