So if someone says an opinion some other poster expresses is all wrong because, for example, "no one on FR has ever said to kill all chickens that lay brown eggs". Later that same person says, "You can't find a single example of anyone who ever said to kill all chickens that lay brown eggs, can you?", (which is pretty much "impeaching the witness" by implication), the proper way to deal with that is to by first going to the thread where such a comment was in fact made.
From that thread, respond to the comment containing "Kill all chickens that . . . " in the original thread, including in the list of recipients the person who denies any such comment exists, as well as others actively involved in the thread and series of comments wherein it was said on such statement exists. Right?
Yes, because then the brown egg laying chicken remark is being argued in the context it was raised.
However, if another poster directly challenges you: "I've never said to kill brown egg laying chickens" then a direct link to his statement is a reasonable reply.