simp only [occursIn]
v : Var
a✝ : String
phi : Formula
phi_ih : occursIn v phi ↔ v ∈ phi.varSet
⊢ occursIn v (forall_ a✝ phi) ↔ v ∈ (forall_ a✝ phi).varSet
v : Var
a✝ : String
phi : Formula
phi_ih : occursIn v phi ↔ v ∈ phi.varSet
⊢ occursIn v phi ↔ v ∈ (forall_ a✝ phi).varSet