Natural This kind of function signifies a partial functionality on naturals, where the proof of even (n) could be witnessed for a precondition. From your Cambridge English Corpus There isn't a actual distinction between the usual initially-order objects in the mathematical discourse - such as naturals, sets and so on https://share.evernote.com/note/74e6f5bf-4e0f-8a5f-1a27-70e011ebb01c