// I AM NOT DONE

    // TODO: add another error conversion function here.

    fn from_parse_int(err: ParseIntError) -> ParsePosNonzeroError {
        ParsePosNonzeroError::ParseInt(err)
    }

    // TODO: change this to return an appropriate error instead of panicking
    // when `parse()` returns an error.
    let x: i64 = s.parse().unwrap();

    let x: i64 = s.parse()
        .map_err(ParsePosNonzeroError::from_parse_int)?;

// I AM NOT DONE

fn main() -> Result<(), ParseIntError> {

fn main() -> Result<(), Box<dyn error::Error>> {

// I AM NOT DONE

use std::cmp::Ordering::{Less, Equal, Greater};

        Ok(PositiveNonzeroInteger(value as u64))

        match value.cmp(&0) {
            Less => Err(CreationError::Negative),
            Equal => Err(CreationError::Zero),
            Greater => Ok(PositiveNonzeroInteger(value as u64)),
        }

// I AM NOT DONE

fn main() {

fn main() -> Result<(), ParseIntError> {

    Ok(())

// I AM NOT DONE

    Ok(qty * cost_per_item + processing_fee)

    Ok(qty? * cost_per_item + processing_fee)

// I AM NOT DONE
pub fn generate_nametag_text(name: String) -> Option<String> {

pub fn generate_nametag_text(name: String) -> Result<String, String> {

        Some(format!("Hi! My name is {}", name))

        Ok(format!("Hi! My name is {}", name))

        None

        Err(String::from("`name` was empty; it must be nonempty."))

            Some("Hi! My name is Beyoncé".into())

            Ok("Hi! My name is Beyoncé".into())