simp
v x : VarName
phi : Formula
phi_ih : isFreeIn v phi ↔ v ∈ phi.freeVarSet
⊢ ¬v = x ∧ isFreeIn v phi ↔ v ∈ phi.freeVarSet \ {x}
v x : VarName
phi : Formula
phi_ih : isFreeIn v phi ↔ v ∈ phi.freeVarSet
⊢ ¬v = x ∧ isFreeIn v phi ↔ v ∈ phi.freeVarSet ∧ ¬v = x