Theorem wundif

Description: A weak universe is closed under class difference. (Contributed by Mario Carneiro, 2-Jan-2017)

	Ref	Expression
Hypotheses	wununi.1	|- ( ph -> U e. WUni )
	wununi.2	|- ( ph -> A e. U )
Assertion	wundif	|- ( ph -> ( A \ B ) e. U )

Proof

Step	Hyp	Ref	Expression
1		wununi.1	|- ( ph -> U e. WUni )
2		wununi.2	|- ( ph -> A e. U )
3		difssd	|- ( ph -> ( A \ B ) C_ A )
4	1 2 3	wunss	|- ( ph -> ( A \ B ) e. U )