Function change 

pub async fn change<E: From<EngineError>, B: Backend>(
    stream: &RefCell<Stream>,
    build: impl FnMut(&Stream) -> Result<Change, E>,
    tier: &Tier<B>,
) -> Result<Changed, E>

build’s change, the stream held only while each try runs, between two blocks: an exact stream pulled once its edit is done plays it where issued. build runs again whenever the stream changed under it.