∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {m : Std.HashSet α} [EquivBEq α] [LawfulHashable α] [inst : Inhabited α]
  {l : List α} {k k' : α},
  (k == k') = true → k ∉ m → List.Pairwise (fun a b => (a == b) = false) l → k ∈ l → (m.insertMany l).get! k' = k