code: func (n *Negroni) Use(handler Handler) {
  	if handler == nil {
  		panic("handler cannot be nil")
  	}
  
  	n.handlers = append(n.handlers, handler)
  	n.middleware = build(n.handlers)
  }
docs: // Use adds a Handler onto the middleware stack. Handlers are invoked in the order they are added to a Negroni.