_toTex (options) {
      const parenthesis =
          (options && options.parenthesis) ? options.parenthesis : 'keep'
      let expr = this.expr.toTex(options)
      if (needParenthesis(this, parenthesis, options && options.implicit)) {
        expr = `\\left(${expr}\\right)`
      }

      return '\\mathrm{' + this.name +
        '}\\left(' + this.params.map(toSymbol).join(',') + '\\right)=' + expr
    }
get LaTeX representation
@param {Object} options
@return {string} str