toTex (options) {
      let customTex
      if (options &&
          (typeof options.handler === 'object') &&
          hasOwnProperty(options.handler, this.name)) {
        // callback is a map of callback functions
        customTex = options.handler[this.name](this, options)
      }

      if (typeof customTex !== 'undefined') {
        return customTex
      }

      // fall back to Node's toTex
      return super.toTex(options)
    }
Get LaTeX representation. (wrapper function)
This overrides parts of Node's toTex function.
If callback is an object containing callbacks, it
calls the correct callback for the current node,
otherwise it falls back to calling Node's toTex
function.

@param {Object} options
@return {string}