∀ {α : Type u} {β : α → Type v} [inst : BEq α] [EquivBEq α] {l₁ l₂ : List ((a : α) × β a)} {k : α},
  Std.Internal.List.DistinctKeys l₁ →
    Std.Internal.List.containsKey k l₂ = true →
      Std.Internal.List.getKey? k (List.filter (fun p => Std.Internal.List.containsKey p.fst l₂) l₁) =
        Std.Internal.List.getKey? k l₁