_toHTML (options) {
      let object = this.object.toHTML(options)
      if (needParenthesis(this.object)) {
        object =
          '<span class="math-parenthesis math-round-parenthesis">(</span>' +
          object +
          '<span class="math-parenthesis math-round-parenthesis">)</span>'
      }

      return object + this.index.toHTML(options)
    }
Get HTML representation
@param {Object} options
@return {string}