User blog comment:Deedlit11/Bignum Bakeoff appetizer/@comment-24061664-20140915184506/@comment-24061664-20140915194451

Found English explanation.

http://stackoverflow.com/questions/2575153/must-declare-function-prototype-in-c

Most people say that it is not required, although recommended. In this case the objective is just to shorten the code, so it's OK.