∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {m : Std.DHashMap α fun x => Unit} [EquivBEq α] [LawfulHashable α]
  {l : List α} {k : α},
  Std.DHashMap.Const.get? (Std.DHashMap.Const.insertManyIfNewUnit m l) k =
    if k ∈ m ∨ l.contains k = true then some () else none