Relation of association.
identifier is a source value: character escapes and character
references are not parsed.
It can match another node.
Its value must be normalized.
To normalize a value, collapse markdown whitespace ([\t\n\r ]+) to a space,
trim the optional initial and/or final space, and perform Unicode-aware
case-folding.
Optional label
Internal relation from one node to another.
Whether the value of
identifieris expected to be a unique identifier or not depends on the type of node including the Association. An example of this is that they should be unique on Definition, whereas multiple LinkReferences can be non-unique to be associated with one definition.