Theorem dfss

Description: Variant of subclass definition df-ss . (Contributed by NM, 21-Jun-1993)

	Ref	Expression
Assertion	dfss	$$⊢A\subseteq B\leftrightarrow A=A\cap B$$

Proof

Step	Hyp	Ref	Expression
1		df-ss	$$⊢A\subseteq B\leftrightarrow A\cap B=A$$
2		eqcom	$$⊢A\cap B=A\leftrightarrow A=A\cap B$$
3	1 2	bitri	$$⊢A\subseteq B\leftrightarrow A=A\cap B$$