code: public static <E> Multiset.Entry<E> immutableEntry(@Nullable E e, int n) {
      return new ImmutableEntry<E>(e, n);
    }
docs: Returns an immutable multiset entry with the specified element and count. The entry will be
  serializable if {@code e} is.
  
  @param e the element to be associated with the returned entry
  @param n the count to be associated with the returned entry
  @throws IllegalArgumentException if {@code n} is negative