Enumerations for _Bond_annotation.Bond_type (An open enumeration)
amide	enumeration_text
covalent	enumeration_text
directed	enumeration_text
disulfide	enumeration_text
ester	enumeration_text
ether	enumeration_text
hydrogen	enumeration_text
metal coordination	enumeration_text
peptide	enumeration_text
thioether	enumeration_text