∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} [inst : EquivBEq α] [inst_1 : LawfulHashable α] [inst_2 : Inhabited α]
  {l : List α} {k k' : α},
  (k == k') = true →
    List.Pairwise (fun a b => (a == b) = false) l → k ∈ l → (Std.ExtHashMap.unitOfList l).getKey! k' = k