The kinetics of the quantitative, symmetry allowed, reverse electron demand cycloadditions of the pseudo-1,3-dipole SNS+with alkynes and nitriles; the preparation and X-ray crystal structures of NCCSNSCHAsF6and SNSNC–CNSNS(AsF6)2: the precursor to a new class of S2N2C–CN2S2n+(n= 0,1,2) bicyclics