[math-fun] I'm so buzzed I can't tell a 6 from a 9.

In[38]:= ChemicalData["Caffeine", "IUPACName"] Out[38]= "1,3,7-trimethylpurine-2,6-dione" agreeing with https://en.wikipedia.org/wiki/Caffeine#Chemistry in both structural formula (ChemicalData["Caffeine"]) and name. Why isn't it 1,3,7-trimethylpurine-2,9-dione? I.e., Why does the imidazole (pentagonal) ring count for the methyls, but not the -ones? —rwg