intro x a1
case h.e'_2
v t : VarName
X : DefName
xs : List VarName
h1 : t ∉ xs
⊢ ∀ x ∈ xs, (if t = if v = x then t else x then v else if v = x then t else x) = x
case h.e'_2
v t : VarName
X : DefName
xs : List VarName
h1 : t ∉ xs
x : VarName
a1 : x ∈ xs
⊢ (if t = if v = x then t else x then v else if v = x then t else x) = x