https://www.drmaciver.com/2006/04/journal-of-obscure-results-1-nedomas-pathology/


X가 Hausdorff space고 거기에 Borel sigma algebra를 줬을때


product measurable space X \times X가 borel set들을 포함하긴 커녕


닫힌 집합인 diagonal도 measurable이 아님ㄷㄷ