public static <E> PCollectionsImmutableSet<E> singleton(E e) {
        return new PCollectionsImmutableSet<>(HashTreePSet.singleton(e));
    }
@param e the element
@return a wrapped hash-based persistent set that has a single element
@param <E> the element type
server-common/src/main/java/org/apache/kafka/server/immutable/pcollections/PCollectionsImmutableSet.java