3-Aminopyrrole-2,5-diones. Part 3. Synthesis of Benzenehexacarboxylic Acid Trimethylimide and 1,1′-Dimethyl-4-methylamino[3,3′]bipyrrolyl-2,5,2′,5′-tetraone by the Reaction of 1-Methyl-3-methylaminopyrrole-2,5-dione with Hydrogen Chloride.