* subclass overrides all the methods that access it here. Thus, all the methods here can safely
     * 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.
     */