Another xref case: example of usage

An xref in a definition should denote the source of the definition - may be a 
person, a meeting, a publication, a class from another ontology.

An xref in a synonym should denote a usage of a label to denote the class, or a 
source of the synonym (these two cases seem indistinguishable from me - e.g. 
the source of a synonym in GO may be a CHEBI class, and this is also an 
instance of usage).

An xref at the level of a Term stanza has no strong semantics. However, 
additional semantics can be provided on a per prefix basis using obof1.4 header 
macros (e.g. treat-xrefs-as-equivalent-to)

An example of usage is typically use of the term in a paper or other narrative, 
so listing of a synonym in Chebi, or biothesaurus would not count as such. 
Perhaps that doesn't matter. 

treat-xrefs-as-equivalent-Tag semantics not yet in spec. Todo.

Providing this documentation in the spec may solve the problem adequately.

treat-xrefs-as-equivalent-Tag added to spec.

The translation rules for xrefs should now be complete. Guidelines for usage 
belong elsewhere