"Where a party without explanation fails to call as a witness a person whom he might reasonably be expected to call, if that person's evidence would be favourable to him, then, although the jury may not treat as evidence what they may as a matter of speculation think that that person would have said if he had been called as a witness, nevertheless it is open to the jury to infer that that person's evidence would not have helped that party's case; if the jury draw that inference, then they may properly take it into account against the party in question for two purposes, namely, (a) in deciding whether to accept any particular evidence, which has in fact been given, either for or against that party, and which relates to a matter with respect to which the person called as a witness could have spoken, and (b) in deciding whether to draw inferences of fact, which are open to them upon evidence which has been given, again in relation to matters with respect to which the person not called as a witness could have spoken."