congr! <;> tauto
v t : VarName
phi psi : Formula
phi_ih : ¬occursIn t phi → fastReplaceFree t v (fastReplaceFree v t phi) = phi
psi_ih : ¬occursIn t psi → fastReplaceFree t v (fastReplaceFree v t psi) = psi
h1 : ¬(occursIn t phi ∨ occursIn t psi)
⊢ (fastReplaceFree t v (fastReplaceFree v t phi)).iff_ (fastReplaceFree t v (fastReplaceFree v t psi)) = phi.iff_ psi
no goals