match b == a with
| true => dsimp; exact htail
| false =>
  dsimp; refine pairwise_cons.mpr <| And.intro ?_ ?_
  . intro _ hb h; cases h
    exact hhead b (List.mem_of_mem_erase hb) rfl
  . exact IH htail
case cons
α : Type u
β : Type v
inst✝ : DecidableEq α
a b : α
l : List α
IH : Nodup l → Nodup (List.erase l a)
hnodup : Nodup (b :: l)
hhead : ∀ (a' : α), a' ∈ l → b ≠ a'
htail : Nodup l
⊢ Nodup
    (match b == a with
    | true => l
    | false => b :: List.erase l a)
no goals