Function writable 

fn writable(store: &Store, path: &Path, sharing: &Sharing<'_>) -> bool

Whether a file may be written where a record says: never through a link below its store’s folder (a link to elsewhere would carry the write out of it), nor over a link, nor inside the sharing folder (it would travel in plain) or Sioul’s own state, but for the stores of that state itself (state/…: the shield’s answers, local lists’ state, the spam filter’s label logs), which live there by design, each in its own folder.