Function error 

pub fn error<I, O, E, R, F>(f: F) -> impl Pipe<I, O, E, R>
where
    F: FnMut() -> FatalError<E>,

creates a pipe that always return an error

#[derive(Debug, PartialEq)]
enum Error {
    Tag(TagStrError),
    Incomplete(Incomplete),
    Other
}

impl From<TagStrError> for Error { fn from(e: TagStrError) -> Error { Error::Tag(e)} }

impl From<Incomplete> for Error { fn from(e: Incomplete) -> Error { Error::Incomplete(e) } }

impl std::error::Error for Error {}

impl std::fmt::Display for Error {
   fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{self:?}")
    }
}


assert_eq!(tag::<Error, _, _>("a").or(error(|| FatalError::Fatal(Error::Other))).apply("b"), Err(FatalError::Fatal(Error::Other)));