assertThrows(NullPointerException.class, () -> collection.retainAll(null));
  }

  private void expectReturnsTrue(Target target) {
    String message = Platform.format("retainAll(%s) should return true", target);
    assertTrue(message, collection.retainAll(target.toRetain));
  }

  private void expectReturnsFalse(Target target) {
    String message = Platform.format("retainAll(%s) should return false", target);

    assertThrows(NullPointerException.class, () -> collection.retainAll(null));
  }

  private void expectReturnsTrue(Target target) {
    String message = Platform.format("retainAll(%s) should return true", target);
    assertTrue(message, collection.retainAll(target.toRetain));
  }

  private void expectReturnsFalse(Target target) {
    String message = Platform.format("retainAll(%s) should return false", target);

      assertTrue(Thread.holdsLock(mutex));
      return delegate.addAll(collection);
    }

    @Override
    public boolean retainAll(Collection<?> collection) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.retainAll(collection);
    }

    @Override
    public void clear() {
      assertTrue(Thread.holdsLock(mutex));
      delegate.clear();
    }

      boolean didRemove = entrySet.retainAll(entriesToRetain);
      assertEquals(shouldRemove, didRemove);
      assertEquals(entriesToRetain.size(), map.size());
      for (Entry<K, V> entry : entriesToRetain) {
        assertTrue(entrySet.contains(entry));
      }
    } else {
      assertThrows(UnsupportedOperationException.class, () -> entrySet.retainAll(entriesToRetain));
    }
    assertInvariants(map);

        }

        @Override
        public boolean removeAll(Collection<?> c) {
          return removeEntriesIf(Maps.<K>keyPredicateOnEntries(in(c)));
        }

        @Override
        public boolean retainAll(Collection<?> c) {
          return removeEntriesIf(Maps.<K>keyPredicateOnEntries(not(in(c))));
        }

        @Override
        public boolean remove(@Nullable Object o) {

      assertTrue(Thread.holdsLock(mutex));
      return delegate.addAll(collection);
    }

    @Override
    public boolean retainAll(Collection<?> collection) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.retainAll(collection);
    }

    @Override
    public void clear() {
      assertTrue(Thread.holdsLock(mutex));
      delegate.clear();
    }

        }

        @Override
        public boolean removeAll(Collection<?> c) {
          return removeEntriesIf(Maps.<K>keyPredicateOnEntries(in(c)));
        }

        @Override
        public boolean retainAll(Collection<?> c) {
          return removeEntriesIf(Maps.<K>keyPredicateOnEntries(not(in(c))));
        }

        @Override
        public boolean remove(@Nullable Object o) {

    return Multisets.removeAllImpl(this, elementsToRemove);
  }

  /**
   * A sensible definition of {@link #retainAll} in terms of the {@code retainAll} method of {@link
   * #elementSet}. If you override {@link #elementSet}, you may wish to override {@link #retainAll}
   * to forward to this implementation.
   *
   * @since 7.0
   */
  @Override

    // Split out all the specified sizes.
    Set<Feature<?>> sizesToTest = Helpers.<Feature<?>>copyToSet(CollectionSize.values());
    sizesToTest.retainAll(features);
    features.removeAll(sizesToTest);

    addImpliedFeatures(sizesToTest);
    sizesToTest.retainAll(asList(CollectionSize.ZERO, CollectionSize.ONE, CollectionSize.SEVERAL));

    logger.fine("   Sizes: " + formatFeatureSet(sizesToTest));

      }
      return changed;
    }

    @Override
    public boolean retainAll(Collection<?> c) {
      checkNotNull(c);
      boolean changed = false;
      Iterator<Map<C, V>> iterator = backingMap.values().iterator();
      while (iterator.hasNext()) {
        Map<C, V> map = iterator.next();
        if (map.keySet().retainAll(c)) {
          changed = true;
          if (map.isEmpty()) {