∀ {α : Type u} {β : α → Type v} [inst : BEq α] [inst_1 : LawfulBEq α] {a : α} {f : Option (β a) → Option (β a)}
  {l : List ((a : α) × β a)},
  Std.Internal.List.DistinctKeys l →
    Std.Internal.List.containsKey a (Std.Internal.List.alterKey a f l) =
      (f (Std.Internal.List.getValueCast? a l)).isSome