case imp_ phi psi phi_ih psi_ih =>
  simp only [Formula.freeVarSet]
  simp only [occursIn]
  simp
  tauto
v : Var
phi psi : Formula
phi_ih : occursIn v phi ∧ v.isFree ↔ v ∈ phi.freeVarSet
psi_ih : occursIn v psi ∧ v.isFree ↔ v ∈ psi.freeVarSet
⊢ occursIn v (phi.imp_ psi) ∧ v.isFree ↔ v ∈ (phi.imp_ psi).freeVarSet
no goals