intro v a1
case h.e'_2.h1
x : VarName
phi : Formula
phi_ih :
  ∀ (σ σ' : VarName → VarName),
    (∀ (v : VarName), isFreeIn v phi → σ v = σ' v) → fastReplaceFree σ phi = fastReplaceFree σ' phi
σ σ' : VarName → VarName
h1 : ∀ (v : VarName), ¬v = x ∧ isFreeIn v phi → σ v = σ' v
⊢ ∀ (v : VarName), isFreeIn v phi → Function.updateITE σ x x v = Function.updateITE σ' x x v
case h.e'_2.h1
x : VarName
phi : Formula
phi_ih :
  ∀ (σ σ' : VarName → VarName),
    (∀ (v : VarName), isFreeIn v phi → σ v = σ' v) → fastReplaceFree σ phi = fastReplaceFree σ' phi
σ σ' : VarName → VarName
h1 : ∀ (v : VarName), ¬v = x ∧ isFreeIn v phi → σ v = σ' v
v : VarName
a1 : isFreeIn v phi
⊢ Function.updateITE σ x x v = Function.updateITE σ' x x v