@Override
    public String toString() {
        return "PCollectionsImmutableSet{" +
            "underlying=" + underlying() +
            '}';
    }
@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