| 540 / | HD | ORGANIC COMPOUNDS -- PART OF THE CLASS 532-570 SERIES | 
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| * | DD | ORGANIC COMPOUNDS (Class 532, Subclass 1) {1} | 
| 1 | DF | .~ HETEROCYCLIC CARBON COMPOUNDS CONTAINING A HETERO RING HAVING CHALCOGEN (I.E., OXYGEN, SULFUR, SELENIUM OR TELLURIUM) OR NITROGEN AS THE ONLY RING HETERO ATOMS {6} | 
| 484 | DF | .~.~ The hetero ring contains seven members including nitrogen and carbon {14} | 
| 544 |  | .~.~.~ The hetero ring contains chalcogen {2} | 
| 545 | DF | .~.~.~.~> Plural nitrogens in the heterocyclic ring | 
| 546 | DF | .~.~.~.~> Polycyclo ring system which contains the hetero ring as one of the cyclos {2} |