Studies in 5-Substituted 6-Azauracils. Part 10. Synthesis of 3-Thioxo-2,3,4,6-tetrahydro-1,2,4-triazino[5,6-f] 2,3,4,6-tetrahydro-1,2,4-triazino15,6-f|naphthyridine-6-one.