src/main/java/org/organicdesign/fp/collections/PersistentTreeSet.java
@Override public Option<E> head() {
        return size() > 0 ? Option.some(impl.firstKey()) : Option.none();
    }
@Override public Option<E> head() {
        return size() > 0 ? Option.some(impl.firstKey()) : Option.none();
    }
{@inheritDoc}