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Metamath Proof Explorer

Theorem sbcovOLD

Description: Obsolete version of sbcov as of 26-Aug-2025. (Contributed by NM, 14-May-1993) (Revised by GG, 7-Aug-2023) (Proof modification is discouraged.) (New usage is discouraged.)

Ref Expression
Assertion sbcovOLD ⊢ y x x y φ ↔ y x φ

Proof

Step Hyp Ref Expression
1 sbcom3vv ⊢ y x x y φ ↔ y x y y φ
2 sbid ⊢ y y φ ↔ φ
3 2 sbbii ⊢ y x y y φ ↔ y x φ
4 1 3 bitri ⊢ y x x y φ ↔ y x φ