assertTrue(Thread.holdsLock(mutex));
      return delegate.removeFirstOccurrence(o);
    }

    @Override
    public boolean removeLastOccurrence(Object o) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.removeLastOccurrence(o);
    }

    @Override
    public void push(E e) {
      assertTrue(Thread.holdsLock(mutex));
      delegate.push(e);
    }

      synchronized (mutex) {
        return delegate().removeFirstOccurrence(o);
      }
    }

    @Override
    public boolean removeLastOccurrence(@CheckForNull Object o) {
      synchronized (mutex) {
        return delegate().removeLastOccurrence(o);
      }
    }

    @Override
    public void push(E e) {
      synchronized (mutex) {
        delegate().push(e);
      }

      synchronized (mutex) {
        return delegate().removeFirstOccurrence(o);
      }
    }

    @Override
    public boolean removeLastOccurrence(@CheckForNull Object o) {
      synchronized (mutex) {
        return delegate().removeLastOccurrence(o);
      }
    }

    @Override
    public void push(E e) {
      synchronized (mutex) {
        delegate().push(e);
      }