simp only [if_neg c1]
case neg
v t : VarName
X : DefName
xs : List VarName
h1 : t ∉ xs
x : VarName
a1 : x ∈ xs
c1 : ¬v = x
⊢ (if t = if v = x then t else x then v else if v = x then t else x) = x
case neg
v t : VarName
X : DefName
xs : List VarName
h1 : t ∉ xs
x : VarName
a1 : x ∈ xs
c1 : ¬v = x
⊢ (if t = x then v else x) = x