[26] That is enough to dispose of the appellant's contention, but since the matter was argued and may arise again it is desirable to discuss the issue of practicability; the appellant's counsel pointed out that no issue as to practicability was raised in the defence. The appellant's counsel relied on the result of Peachey in this connection, but there the argument relating to practicability was abandoned. Here, if the judge had held that the injury, sustained in the way sworn to by Muller, showed a prima facie breach of s 9(1) of the Act, the appellant's counsel below could hardly have succeeded on the basis that practicability was not mentioned in the respondents' pleading; that was so because the incident mentioned by Muller could not be identified with that pleaded by the appellant, nor even with that sworn to in the appellant's evidence. It would in my opinion have been necessary, strictly speaking, if the appellant wished to base an alternative case on Muller's evidence, for him to seek leave to amend his pleading; that would have given the respondents an opportunity to amend in response and, if they chose, to call further evidence. Of course, the matter might have proceeded less formally, without an amendment, the parties concurring in litigating on the basis that the plaintiff was running an alternative case on the Muller account; plainly, that did not happen. The expert evidence, for example, was directed to the appellant's account; the expert witnesses were not questioned by counsel for the respondents about the changes which acceptance of Muller's version would have made to their opinions about the safety of the operation.