You should first explain in what manner nations, and the UK courts in particular, incorporate customary rules. You must of course make reference to the relevant case law. Secondly, you must explain the rationale for incorporation as opposed to transformation which otherwise applies in respect of treaties (ie that it would be unjust not to recognize the existence of a [customary] rule that already binds the State in question and which has an effect on individual rights and obligations and which in any case is unlikely to be codified). Thirdly, you must demonstrate the difficulties facing the courts in respect of crimes under customary law and mention that UK courts are disinclined to incorporate all customary crimes for fear of infringing fundamental rights, such as the prohibition against retroactive legislation.