I've notice there is "merde" method which is available in the example like in https://github.com/bnjbvr/rouille/blob/7e9523fe24026bff1a3a7...

Merde is an alias for "calisse" and "oups" , see https://github.com/bnjbvr/rouille/blob/7e9523fe24026bff1a3a7...