∀ {α : Type u} [inst : BEq α] [PartialEquivBEq α] {l : List ((_ : α) × Unit)} {toInsert : List α} {k : α},
  toInsert.contains k = false →
    Std.Internal.List.containsKey k (Std.Internal.List.insertListIfNewUnit l toInsert) = true →
      Std.Internal.List.containsKey k l = true