src/main/java/org/organicdesign/fp/collections/PersistentTreeSet.java
@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}