Fair enough, though there weren't really any "aspersions" cast. I'm aware you can unravel set theory if you care enough about that particular exercise. But, in

Fair enough, though there weren't really any "aspersions" cast. I'm aware you can unravel set theory if you care enough about that particular exercise. But, in context, you could also interpret that comment as being written by someone who isn't aware that the empty set is defined as {}. Responding to someone's definition that natural 0 = {} that mathematics is a "castle built on sand" isn't exactly a cogent criticism of set theory.