(b) A sues B for a libel imputing disgraceful conduct to A; B affirms that the matter alleged to be libellous is true. The position and relations of the parties at the time when the libel was published may be relevant facts as introductory to the facts in issue. The particulars of a dispute between A and B about a matter unconnected with the alleged libel are irrelevant, though the fact that there was a dispute may be relevant if it affected the relations between A and B.