checkNonnegative(count, "count");
    int oldCount = (count == 0) ? backingMap.remove(element) : backingMap.put(element, count);
    size += (count - oldCount);
    return oldCount;
  }

  @Override
  public final boolean setCount(@ParametricNullness E element, int oldCount, int newCount) {
    checkNonnegative(oldCount, "oldCount");
    checkNonnegative(newCount, "newCount");

  private void assertSetCountNegativeOldCount() {
    try {
      getMultiset().setCount(e3(), -1, 1);
      fail("calling setCount() with a negative oldCount should throw IllegalArgumentException");
    } catch (IllegalArgumentException expected) {
    }
  }

  // Negative oldCount.

  @CollectionFeature.Require(SUPPORTS_ADD)
  public void testSetCountConditional_negativeOldCount_addSupported() {
    assertSetCountNegativeOldCount();

  private void assertSetCountNegativeOldCount() {
    try {
      getMultiset().setCount(e3(), -1, 1);
      fail("calling setCount() with a negative oldCount should throw IllegalArgumentException");
    } catch (IllegalArgumentException expected) {
    }
  }

  // Negative oldCount.

  @CollectionFeature.Require(SUPPORTS_ADD)
  public void testSetCountConditional_negativeOldCount_addSupported() {
    assertSetCountNegativeOldCount();

    return setCountImpl(this, element, count);
  }

  @CanIgnoreReturnValue
  @Override
  public boolean setCount(@ParametricNullness E element, int oldCount, int newCount) {
    return setCountImpl(this, element, oldCount, newCount);
  }

  // Bulk Operations

  /**
   * {@inheritDoc}
   *
   * <p>This implementation is highly efficient when {@code elementsToAdd} is itself a {@link

    return setCountImpl(this, element, count);
  }

  @CanIgnoreReturnValue
  @Override
  public boolean setCount(@ParametricNullness E element, int oldCount, int newCount) {
    return setCountImpl(this, element, oldCount, newCount);
  }

  // Bulk Operations

  /**
   * {@inheritDoc}
   *
   * <p>This implementation is highly efficient when {@code elementsToAdd} is itself a {@link

    }
  }

  @CollectionFeature.Require(SUPPORTS_REMOVE)
  public void testRemove_occurrences_0() {
    int oldCount = getMultiset().count(e0());
    assertEquals(
        "multiset.remove(E, 0) didn't return the old count",
        oldCount,
        getMultiset().remove(e0(), 0));
  }

  @CollectionFeature.Require(SUPPORTS_REMOVE)
  public void testRemove_occurrences_negative() {