simp only [← List.mem_toFinset]
case h1.h1
D : Type
I : Interpretation D
v t : VarName
F F' : Formula
h1_X : DefName
h1_xs : List VarName
h1_v h1_t : VarName
V : VarAssignment D
hd : Definition
tl : List Definition
ih :
  Holds D I (Function.updateITE V h1_v (V h1_t)) tl (def_ h1_X h1_xs) ↔
    Holds D I V tl (def_ h1_X (List.map (fun x => if h1_v = x then h1_t else x) h1_xs))
c1 : h1_X = hd.name ∧ h1_xs.length = hd.args.length
c2 : h1_X = hd.name ∧ (List.map (fun x => if h1_v = x then h1_t else x) h1_xs).length = hd.args.length
v' : VarName
s1 : List.map (Function.updateITE V h1_v (V h1_t)) h1_xs = List.map (V ∘ fun x => if h1_v = x then h1_t else x) h1_xs
a1 : v' ∈ hd.q.freeVarSet
⊢ v' ∈ hd.args
case h1.h1
D : Type
I : Interpretation D
v t : VarName
F F' : Formula
h1_X : DefName
h1_xs : List VarName
h1_v h1_t : VarName
V : VarAssignment D
hd : Definition
tl : List Definition
ih :
  Holds D I (Function.updateITE V h1_v (V h1_t)) tl (def_ h1_X h1_xs) ↔
    Holds D I V tl (def_ h1_X (List.map (fun x => if h1_v = x then h1_t else x) h1_xs))
c1 : h1_X = hd.name ∧ h1_xs.length = hd.args.length
c2 : h1_X = hd.name ∧ (List.map (fun x => if h1_v = x then h1_t else x) h1_xs).length = hd.args.length
v' : VarName
s1 : List.map (Function.updateITE V h1_v (V h1_t)) h1_xs = List.map (V ∘ fun x => if h1_v = x then h1_t else x) h1_xs
a1 : v' ∈ hd.q.freeVarSet
⊢ v' ∈ hd.args.toFinset