I would like to add my thoughts to this one. I agree that the symbol in question is not your "typical" plus sign in a circle. However, it IS called a "direct sum" in addition to the "bun" name. Unless the text actually says "bun", I think it would be clearer to the reader to acknowledge that it is a type of sum and do so using the shape indicator and ds rather than bn. Just another thought to add to the mix. We think we've seen it all until the next one comes along!