4. The question whether such a departure would be unconscionable relates to the conduct of the allegedly estopped party in all the circumstances. That party must have played such a part in the adoption of, or persistence in, the assumption that he would be guilty of unjust and oppressive conduct if he were now to depart from it. The cases indicate four main, but not exhaustive, categories in which an affirmative answer to that question may be justified, namely, where that party: (a) has induced the assumption by express or implied representation; (b) has entered into contractual or other material relations with the other party on the conventional basis of the assumption; (c) has exercised against the other party rights which would exist only if the assumption were correct; (d) knew that the other party laboured under the assumption and refrained from correcting him when it was his duty in conscience to do so. Ultimately, however, the question whether departure from the assumption would be unconscionable must be resolved not by reference to some preconceived formula framed to serve as a universal yardstick but by reference to all the circumstances of the case, including the reasonableness of the conduct of the other party in acting upon the assumption and the nature and extent of the detriment which he would sustain by acting upon the assumption if departure from the assumed state of affairs were permitted. In cases falling within category (a), a critical consideration will commonly be that the allegedly estopped party knew or intended or clearly ought to have known that the other party would be induced by his conduct to adopt, and act on the basis of, the assumption. Particularly in cases falling within category (b), actual belief in the correctness of the fact or state of affairs assumed may not be necessary. Obviously, the facts of a particular case may be such that it falls within more than one of the above categories."