case zero =>
  simp only [shift]
  simp only [Var.openList]
  simp
D : Type
I : Interpretation D
V : VarAssignment D
F : Formula
z : String
h1 : ¬occursFreeIn (free_ z) F
d : D
a1 : occursFreeIn (bound_ 0) F
⊢ shift D V d (bound_ 0) =
    if Var.openList 0 [free_ z] (bound_ 0) = free_ z then d else V (Var.openList 0 [free_ z] (bound_ 0))
no goals