SEARCH Articles Figures Tables Compounds containing Three or Four Fused Heterocyclic Rings (5,5,5), (5,5,6), (5,5,7), (5,6,7), and Condensed 1,2,4-triazines: I. Fused heterocycles with three-, four-, and fivemembered rings Fused rings Polycyclic Hydrocarbons, Nonalternant Compounds with Four Fused Rings