Photochromic and thermochromic compounds. I. Structures of (E) and (Z) isomers of 2-isopropylidene-3-[1-(2-methyl-5-phenyl-3-thienyl)ethylidene]succinic anhydride, C20H18O3S, and the photoproduct 7,7a-dihydro-4,7,7,7a-tetramethyl-2-phenylbenzo[b]thiophene-5,6-dicarboxylic anhydride (P), C20H18O3S