HashPMap<K, V> underlying() {
        return underlying;
    }
@param key the key
@param value the value
@return a wrapped hash-based persistent map that has a single mapping
@param <K> the key type
@param <V> the value type
server-common/src/main/java/org/apache/kafka/server/immutable/pcollections/PCollectionsImmutableMap.java