∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {m : Std.ExtDHashMap α fun x => Unit} [inst : EquivBEq α]
  [inst_1 : LawfulHashable α] {l : List α} {k : α},
  k ∈ m → (Std.ExtDHashMap.Const.insertManyIfNewUnit m l).getKey? k = m.getKey? k