@Override public PersistentTreeSet<E> without(E key) {
        return (impl.containsKey(key)) ? new PersistentTreeSet<>(impl.without(key))
                                       : this;
    }
@Override public PersistentTreeSet<E> without(E key) {
        return (impl.containsKey(key)) ? new PersistentTreeSet<>(impl.without(key))
                                       : this;
    }
@ Override public PersistentTreeSet < E > without ( E key ) { return ( impl . containsKey ( key ) ) ? new PersistentTreeSet <> ( impl . without ( key ) ) : this ; }
{@inheritDoc}
{