1,3-Dipolar Cycloaddition of 2-Diazopropane to 1-Phenyl-bis-s-triazolo[4,3-b:3′,4′-f]pyridazine. The Synthesis of 11,11-Dimethyl-11H-pyrazolo[3,4-d]bis-s-triazolo[4,3-b:3′4′-f]pyridazines, Derivatives of a Novel Heterocyclic System.