code: func validateHandler(vfn reflect.Value, ctxType reflect.Type) {
  	var req *Request
  	var resp func() ResponseWriter
  	if !isValidHandler(vfn, ctxType, reflect.TypeOf(resp).Out(0), reflect.TypeOf(req)) {
  		panic(instructiveMessage(vfn, "a handler", "handler", "rw web.ResponseWriter, req *web.Request", ctxType))
  	}
  }
docs: // Panics unless fn is a proper handler wrt ctxType
  // eg, func(ctx *ctxType, writer, request)