[chemical substances]
   compounds
     Class esters [acyl-XR, etc.; R'kE(=O)l(OH)m (l≠0; ≥1 (OH):=(XR)), etc.; X=O,S,Se,Te,etc.; R≠H]
       Class ortho esters [RC(OR')3 (R'≠H), C(OR')4 (R'≠H)]