code: public static <K, V> ImmutableListMultimap<K, V> copyOf(
        Multimap<? extends K, ? extends V> multimap) {
      if (multimap.isEmpty()) {
        return of();
      }
  
      // TODO(lowasser): copy ImmutableSetMultimap by using asList() on the sets
      if (multimap instanceof ImmutableListMultimap) {
        @SuppressWarnings("unchecked") // safe since multimap is not writable
        ImmutableListMultimap<K, V> kvMultimap = (ImmutableListMultimap<K, V>) multimap;
        if (!kvMultimap.isPartialView()) {
          return kvMultimap;
        }
      }
  
      return fromMapEntries(multimap.asMap().entrySet(), null);
    }
docs: Returns an immutable multimap containing the same mappings as {@code multimap}. The generated
  multimap's key and value orderings correspond to the iteration ordering of the {@code
  multimap.asMap()} view.
  
  <p>Despite the method name, this method attempts to avoid actually copying the data when it is
  safe to do so. The exact circumstances under which a copy will or will not be performed are
  undocumented and subject to change.
  
  @throws NullPointerException if any key or value in {@code multimap} is null