Hanasusanto et al. [HKW] pointed out a mistake in our paper. To be precise, the error occurs in the rst formula on page 429. We gratefully acknowledge their careful reading of our paper. We repair the error here in this corrigendum. We have chosen to completely rewrite Section 3.2 of the paper. We have done this so that the correction can be read almost independently of the rest of the paper, and to take the opportunity to improve the exposition.