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Corbyn's been around long enough to know this sort of rhetorical trap
Yet not, apparently, long enough to have developed a way of dealing with it. He didn't have the figures, he didn't cover that by making a joke of it, he didn't challenge the question as a rhetorical trap, he didn't challenge the interviewer, he didn't divert the line of questioning or the subject.
None of those would have necessarily had any more substance than "Er, it will cost, er" but they might have sounded better.
This is the specificity fallacy - asking a question that requires very specific facts or figures to answer. And once those figures are given, the interviewer can say, 'but isn't your figure too low? Aren't you not really thinking this through?' The one being questioned is fucked either way. Corbyn's been around long enough to know this sort of rhetorical trap (which is, by the way, a big part of public school education and yet completely missing from the national curriculum - can't let the proles know what we're up to, haw haw)