fact: Definition unthree_digit (c2 c1 c0 : ascii) : option ascii := let doa := digit_of_ascii in match doa c2, doa c1, doa c0 with | Some n2, Some n1, Some n0 => Some (ascii_of_nat (n2 * 100 + n1 * 10 + n0)) | _, _, _ => None end.
imports: Coq(Ascii Basics Decimal List String ZArith)