Addition reactions of heterocyclic compounds. Part LXIII. New structures for some 2 : 1 molar adducts from dimethyl acetylenedicarboxylate with thiazoles and benzo-imidazoles, -oxazoles, and -thiazoles formed by novel rearrangement. Crystal and molecular structure determinations for tetramethyl 3,8a-dimethylpyrido[2,1-b]thiazole-5,6,7,8-tetracarboxylate and tetramethyl 5-methylpyrido[2,1-b]thiazole-6,7,8,8a-tetracarboxylate