* values are merged using the specified merging function. If the merging function returns {@code
   * null}, then the collector removes the value that has been computed for the key thus far (though
   * future occurrences of the key would reinsert it).
   *
   * <p>Entries will appear in the encounter order of the first occurrence of the key.
   *
   * @since 33.2.0 (available since 21.0 in guava-jre)
   */

     * assume that this field is non-null.
     */
    @CheckForNull ObjectCountHashMap<E> contents;

    /**
     * If build() has been called on the current contents multiset, we need to copy it on any future
     * modifications, or we'll modify the already-built ImmutableMultiset.
     */
    boolean buildInvoked = false;
    /**