is there a difference between countable unions and union between two arbitrary sets ?
https://math.stackexchange.com/questions/3976007/difference-between-countable-unions-and-union-of-any-two-arbitrary-sets-in-the-c
https://math.stackexchange.com/questions/3976007/difference-between-countable-unions-and-union-of-any-two-arbitrary-sets-in-the-c
