code: func (r *Router) FilterParam(param string, filter http.HandlerFunc) {
  	r.Filter(func(w http.ResponseWriter, req *http.Request) {
  		c := NewContext(req)
  		if len(c.Params.Get(param)) > 0 { filter(w, req) }
  	})
  }
docs: // FilterParam adds the middleware filter iff the URL parameter exists.