private void OnNext(IChangeSet<T> changes)
{
    using var @lock = SwappableLock.CreateAndEnter(UpstreamSynchronizationGate);
    
    foreach (var change in changes)
    {
        ...
    }
    
    var downstreamChanges = ChangeSet<T>.Empty;
    if (_downstreamChangesBuffer.Count is not 0)
    {
        downstreamChanges = new ChangeSet<T>(_downstreamChangesBuffer);
        _downstreamChangesBuffer.Clear();
    }
 
    if ((downstreamChanges.Count is not 0) || !_suppressEmptyChangeSets)
    {
        @lock.SwapTo(DownstreamSynchronizationGate);
        
        _downstreamObserver.OnNext(downstreamChanges);
    }
}