∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {m : Std.HashMap α Unit} [EquivBEq α] [LawfulHashable α] {l : List α}
  {k k' : α},
  (k == k') = true →
    k ∉ m → List.Pairwise (fun a b => (a == b) = false) l → k ∈ l → (m.insertManyIfNewUnit l).getKey? k' = some k