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pub fn greet(name: Option<&str>) -> String {
    match name {
        Some(name) => format!("Hello, {}!", name),
        None => "Hello, world!".to_string(),
    }
}

#[cfg(test)]
mod tests {
    use super::greet;

    #[test]
    fn greet_takes_a_none_and_returns_the_default_greeting() {
        assert_eq!("Hello, world!", greet(None));
    }

    #[test]
    fn greet_takes_a_name_and_returns_a_customized_greeting() {
        assert_eq!("Hello, intrepion!", greet(Some("intrepion")));
    }
}