∀ {α : Type u} {β : α → Type v} [inst : BEq α] [EquivBEq α] {l₁ : List ((a : α) × β a)} {l₂ : List α} {k : α},
  Std.Internal.List.DistinctKeys l₁ →
    Std.Internal.List.getKey? k (List.filter (fun p => !l₂.contains p.fst) l₁) =
      if l₂.contains k = true then none else Std.Internal.List.getKey? k l₁