simp only [shift]
D : Type
V : VarAssignment D
hd : String
tl : List String
ih : V ∘ Var.openList 0 (List.map free_ tl) = shiftList D V (List.map (V ∘ free_) tl)
x : String
⊢ V (Var.openList 0 (free_ hd :: List.map free_ tl) (free_ x)) =
    shift D (shiftList D V (List.map (V ∘ free_) tl)) (V (free_ hd)) (free_ x)
D : Type
V : VarAssignment D
hd : String
tl : List String
ih : V ∘ Var.openList 0 (List.map free_ tl) = shiftList D V (List.map (V ∘ free_) tl)
x : String
⊢ V (Var.openList 0 (free_ hd :: List.map free_ tl) (free_ x)) = shiftList D V (List.map (V ∘ free_) tl) (free_ x)