Despite the central role that SMILES plays in cheminformatics, public descriptions of the language remain imprecise. A 1987 report to the US Environmental Protection Agency by Weininger described a language similar to SMILES, but with important differences . paper offered an overview of the language, but omitted or only partially addressed points crucial to correctly implementing SMILES, including: (1) stereochemical configuration; (2) double bond conformation; (3) the encoding and decoding of "aromatic" features; (4) the computation of implicit hydrogen count, especially in the context of aromatic features; syntax; constraints on quantities such as mass number and charge; and (7) enumeration of error states.