Discuss Art 38 ICJ Statute and refer to its drafting history, namely that it is identical to the respective provision of the PCIJ Statute. This should lead to a discussion as to whether this is adequate in the twenty-first century. First, you have to ascertain whether the provision, including the sources of international law was meant to be exhaustive or simply indicative. Then, you must consider which of the sources seem redundant nowadays, eg general principles of law and which could take their place (eg UN General Assembly resolutions, unilateral acts). Your answer must take into account the nature of the international legal system. You have to question whether international society, predominantly States, are ready to accept any other formal source besides treaty and custom. And you have to think of whether the new sources can be subsumed within the existing ones.