split
D : Type
V : VarAssignment D
i : ℕ
⊢ V (if i < 0 then bound_ i else if x : i - 0 < [].length then [][i - 0] else bound_ (i - 0 - [].length + 0)) =
    V (bound_ i)
case inl
D : Type
V : VarAssignment D
i : ℕ
h✝ : i < 0
⊢ V (bound_ i) = V (bound_ i)

case inr
D : Type
V : VarAssignment D
i : ℕ
h✝ : ¬i < 0
⊢ V (if x : i - 0 < [].length then [][i - 0] else bound_ (i - 0 - [].length + 0)) = V (bound_ i)