Enumerations for _Chem_comp_identifier.Type (An open enumeration)
`CAS REGISTRY NUMBER` | 'Chemical Abstracts Registry Number'
`COMMON NAME` | 'Common chemical name'
`MDL Identifier` | 'Molecular Design Limited Identifier'
`PUBCHEM CID` | 'PubChem compound identifier'
`PUBCHEM Identifier` | 'PubChem accession number'
`PUBCHEM SID` | 'PubChem substance identifier'
`SYSTEMATIC NAME` | 'Systematic chemical name'