The section proceeds on the footing that the court intends to reduce the sentence because the offender has expressed a willingness to co-operate with law enforcement agencies in future proceedings. It must be read with s 16A of the Crimes Act which requires the court to take into account a number of matters, including the fact that the offender has co-operated with law enforcement agencies and the fact that the offender has pleaded guilty. By reason of s 21E, it is open to the court to have regard also to the fact that the offender has undertaken to co-operate in the future with law enforcement agencies. It is plain that one of the purposes of s 21E is to provide some kind of sanction to ensure that such undertakings are honoured. If the sentence is to be reduced by reason of an undertaking to give assistance in the future, s 21E requires the judge to specify that the sentence is being reduced for that reason and to state the sentence that would have been imposed but for the reduction. The court must also indicate if the non-parole period is being reduced and, if so, state what the period would have been but for the reduction. If the promised co-operation is not forthcoming, the Director of Public Prosecutions may appeal and the court must then, depending on what is established, substitute the sentence which would have been ordered but for the reduction or substitute another sentence not exceeding the sentence that would have been imposed. The operation of s 21E, therefore, requires the court to specify both the reduction and the sentence which would have been imposed but for the reduction. It may do so both in respect of the head sentence and the non-parole period.