exact phi_ih σ
case h.e'_3
c : Char
phi psi : Formula
phi_ih : ∀ (σ : VarName → VarName), (sub σ c phi).freeVarSet = Finset.image σ phi.freeVarSet
psi_ih : ∀ (σ : VarName → VarName), (sub σ c psi).freeVarSet = Finset.image σ psi.freeVarSet
σ : VarName → VarName
⊢ (sub σ c phi).freeVarSet = Finset.image σ phi.freeVarSet
no goals