Nitrenium Ions. Part 1. Acids-Catalyzed Reactions of 2-Methylindole with Nitrosobenzenes. Crystal Structures of 2-Phenylamino-3- phenylimino-3H-indole, 2-(o-Tolylamino)-3-(o-tolylimino)-3H-indole, N- Phenyl-N-(2-phenylamino-3H-indol-3