Javadoc
A reference to a convention.
There is no controlled vocabulary for conventions, but the author must ensure that the semantics are openly available and that there are mechanisms for implementation. The convention is inherited by all the subelements,
so that a convention for molecule would by default extend to its bond and atom children. This can be overwritten
if necessary by an explicit convention.
It may be useful to create conventions with namespaces (e.g. iupac:name).
Use of convention will normally require non-STMML semantics, and should be used with
caution. We would expect that conventions prefixed with "ISO" would be useful,
such as ISO8601 for dateTimes.
There is no default, but the conventions of STMML or the related language (e.g. CML) will be assumed.