induction xs
P : Formula
xs : List VarName
x : VarName
⊢ isBoundIn x (List.foldr forall_ P xs) ↔ x ∈ xs ∨ isBoundIn x P
case nil
P : Formula
x : VarName
⊢ isBoundIn x (List.foldr forall_ P []) ↔ x ∈ [] ∨ isBoundIn x P

case cons
P : Formula
x head✝ : VarName
tail✝ : List VarName
tail_ih✝ : isBoundIn x (List.foldr forall_ P tail✝) ↔ x ∈ tail✝ ∨ isBoundIn x P
⊢ isBoundIn x (List.foldr forall_ P (head✝ :: tail✝)) ↔ x ∈ head✝ :: tail✝ ∨ isBoundIn x P