simp only [← h1]
case s1.x
α : Type
inst✝ : DecidableEq α
ys : List α
h1 : [].length = ys.length
h2 : [].Nodup
h3 : ys.Nodup
⊢ ys.length = 0
case s1.x
α : Type
inst✝ : DecidableEq α
ys : List α
h1 : [].length = ys.length
h2 : [].Nodup
h3 : ys.Nodup
⊢ [].length = 0