apply Iff.intro
S : Stmt
s t : State
⊢ (Stmt.skip; S, s) ⟹ t ↔ (S, s) ⟹ t
case mp
S : Stmt
s t : State
⊢ (Stmt.skip; S, s) ⟹ t → (S, s) ⟹ t

case mpr
S : Stmt
s t : State
⊢ (S, s) ⟹ t → (Stmt.skip; S, s) ⟹ t