Function case 

pub async fn case<'a, I, W, P, S, E>(
    word: W,
    arms: I,
    env: &mut E,
) -> Result<BoxFuture<'static, ExitStatus>, S::Error>
where
    I: Iterator<Item = PatternBodyPair<&'a [P], S>>,
    W: WordEval<E>,
    P: 'a + WordEval<E>,
    P::Error: IsFatalError,
    S: Spawn<E>,
    S::Error: From<W::Error> + From<P::Error>,
    E: ?Sized + LastStatusEnvironment + ReportErrorEnvironment,

Spawns a case commands from a word to match number of case arms.

First the provided word will be evaluated and compared to each pattern of each case arm. The first arm which contains a pattern that matches the word will have its (and only its) body evaluated.

If no arms are matched, the case command will exit successfully.