code: func (m *RouteMux) FilterParam(param string, filter http.HandlerFunc) {
  	if !strings.HasPrefix(param,":") {
  		param = ":"+param
  	}
  
  	m.Filter(func(w http.ResponseWriter, r *http.Request) {
  		p := r.URL.Query().Get(param)
  		if len(p) > 0 { filter(w, r) }
  	})
  }
docs: // FilterParam adds the middleware filter iff the REST URL parameter exists.