Theorem sbequiOLD

Description: Obsolete version of sbequ as of 10-Aug-2026, as per conventions , paragraph "Biconditional". (Contributed by NM, 14-May-1993) (Proof shortened by Wolf Lammen, 15-Sep-2018) (Proof shortened by Steven Nguyen, 7-Jul-2023) (Proof modification is discouraged.) (New usage is discouraged.)

	Ref	Expression
Assertion	sbequiOLD	⊢ ( 𝑥 = 𝑦 → ( [ 𝑥 / 𝑧 ] 𝜑 → [ 𝑦 / 𝑧 ] 𝜑 ) )

Proof

Step	Hyp	Ref	Expression
1		sbequ	⊢ ( 𝑥 = 𝑦 → ( [ 𝑥 / 𝑧 ] 𝜑 ↔ [ 𝑦 / 𝑧 ] 𝜑 ) )
2	1	biimpd	⊢ ( 𝑥 = 𝑦 → ( [ 𝑥 / 𝑧 ] 𝜑 → [ 𝑦 / 𝑧 ] 𝜑 ) )