 What is more, O may challenge a statement P \(n:\mathbb{N}\) by requesting (O \(\rqst n\)) that P provides one of the accepted nominal definitions above (P \(s(s(\ldots s(0)\ldots )) \equiv_{df} n : \mathbb{N}\)). However, P may provide such a nominal definition only if O has previously stated the definiens (here \(s(s(\ldots s(0)\ldots ))\)).