“For the purpose of any reference in this Part, however expressed, to the term of imprisonment to which a person has been sentenced or which, or part of which, he has served, consecutive terms and terms which are wholly or partly concurrent shall be treated as a single term if – (a) the sentences were passed on the same occasion; or (b) where they were passed on different occasions, the person has not been released under this Part [of the Act] at any time during the period beginning with the first and ending with the last of those occasions.”