code: func (r *Router) OptionsHandler(fn interface{}) *Router {
  	if r.parent != nil {
  		panic("You can only set an OptionsHandler on the root router.")
  	}
  	vfn := reflect.ValueOf(fn)
  	validateOptionsHandler(vfn, r.contextType)
  	r.optionsHandler = vfn
  	return r
  }
docs: // OptionsHandler sets the specified function as the options handler and returns the router.
  // Note that only the root router can have a OptionsHandler handler.