simp
v t x : VarName
phi : Formula
phi_ih : ¬isFreeIn v phi → fastReplaceFree v t phi = phi
h1 : ¬(¬v = x ∧ isFreeIn v phi)
⊢ (if v = x then exists_ x phi else exists_ x (fastReplaceFree v t phi)) = exists_ x phi
v t x : VarName
phi : Formula
phi_ih : ¬isFreeIn v phi → fastReplaceFree v t phi = phi
h1 : ¬(¬v = x ∧ isFreeIn v phi)
⊢ ¬v = x → fastReplaceFree v t phi = phi