Theorem pssss

Description: A proper subclass is a subclass. Theorem 10 of Suppes p. 23. (Contributed by NM, 7-Feb-1996)

	Ref	Expression
Assertion	pssss	$$⊢A\subset B\to A\subseteq B$$

Proof

Step	Hyp	Ref	Expression
1		df-pss	$$⊢A\subset B\leftrightarrow \left(A\subseteq B\wedge A\ne B\right)$$
2	1	simplbi	$$⊢A\subset B\to A\subseteq B$$