We thank James Humphreys for his writing in general, and his correspondence in particular. A.B. acknowledges support from the EU CIG grant UE-14-GT5LD2013-618459, the Asturias Government grant FC-15-GRUPIN14-108 and Spanish Government grant MINECO16-FPA2015-63667-P.J.T. thanks the High Energy Physics Theory Group of the University of Oviedo for their warm hospitality and acknowledges support from the grant ANR-13-BS05-0001. We thank Xavier Bekaert, Nicolas Boulanger and Hugh Osborn for comments on the first version of this paper.