CHEM_LINK group of categories.
Chemical components may be linked together according to component type. Component type
is an enumerated list of common polymer linking types defined in
category CHEM_COMP. By defining linking according
to type rather than component identifier permits a single specification
of generic polymer linkages (i.e. L-peptide linking) between a class
of monomer components.
Linkages which span entities are described in the ENTITY_LINK category.
The geometry of these covalent linkages is also specified in the
CHEM_LINK categories.