simp
D : Type
V : VarAssignment D
j : ℕ
z : String
d : D
x : String
⊢ shift D V d (if free_ x = free_ z then bound_ (j + 1) else free_ x) =
    V (if free_ x = free_ z then bound_ j else free_ x)
D : Type
V : VarAssignment D
j : ℕ
z : String
d : D
x : String
⊢ shift D V d (if x = z then bound_ (j + 1) else free_ x) = V (if x = z then bound_ j else free_ x)