∀ {α : Type u} {β : α → Type v} [inst : BEq α] [inst_1 : LawfulBEq α] {f : (a : α) → β a → Bool}
  {l : List ((a : α) × β a)} {k : α},
  Std.Internal.List.DistinctKeys l →
    Std.Internal.List.getValueCast? k (List.filter (fun p => f p.fst p.snd) l) =
      Option.filter (f k) (Std.Internal.List.getValueCast? k l)