∀ {α : Type u} {β : α → Type v} [inst : BEq α] [PartialEquivBEq α] {l toInsert : List ((a : α) × β a)} {k : α},
  Std.Internal.List.containsKey k (Std.Internal.List.insertList l toInsert) = true →
    (List.map Sigma.fst toInsert).contains k = false → Std.Internal.List.containsKey k l = true