Function in_step 

fn in_step<T>(
    memory: &Path,
    folder: &Path,
    account: &str,
    change: impl FnOnce(&mut State) -> T,
) -> Option<T>

A change to the state under the step’s lock (mirror.step), over the state as it is then, saved: no step runs meanwhile, nor saves over it. None, nothing changed, when Sioul no longer keeps that folder with that account.