code: func (mx *Mux) updateSubRoutes(fn func(subMux *Mux)) {
  	for _, r := range mx.tree.routes() {
  		subMux, ok := r.SubRoutes.(*Mux)
  		if !ok {
  			continue
  		}
  		fn(subMux)
  	}
  }
docs: // Recursively update data on child routers.