You don’t have to be a rocket scientist to spot the pitfalls lying in wait for Francis Poulenc’s 1956 opera: this tale of the death by guillotine of 16 nuns could be voyeuristic, irrelevant, preachy, sentimental – or (possibly) transcendently ­powerful and moving. Opera production is a delicate matter, rarely more so than with this product of a composer devoted to the ­flippant, whose sudden access of operatic piety can easily prompt scorn.Also, how do you deal with the idea of martyrdom in times when it has been appropriated by murderous delusionaries? It feels almost startling that Robert Carsen’s staging doesn’t confront this – but it is the product of what seems a lost age of innocence, having been wandering the world since 1997 and only now