Thiadiazolium ylides: Substituted 2H-1,3,5-thiadiazines and 1,4,5-trisubstituted-imidazoles from 1,2,4- and 1,2,5-thiadiazolium-2-unsubstituted methanide (ylide) systems: ring expansions and ring interconversions via sulfur-nitrogen heterotriene intermediates. Mechanistic ab initio calculations. Azolium 1,3-dipoles