apply Iff.intro
Ss : List Stmt
s t : State
⊢ (Stmt.choice Ss, s) ⟹ t ↔ ∃ i hless, (Ss[i], s) ⟹ t
case mp
Ss : List Stmt
s t : State
⊢ (Stmt.choice Ss, s) ⟹ t → ∃ i hless, (Ss[i], s) ⟹ t

case mpr
Ss : List Stmt
s t : State
⊢ (∃ i hless, (Ss[i], s) ⟹ t) → (Stmt.choice Ss, s) ⟹ t