Notes on custom restriction type

If in the long term we would need to implement a custom restriction entity, we should start from the evaluation of its components in 

https://www.w3.org/TR/2012/REC-xmlschema11-2-20120405/datatypes.html#defn-coss

The benefits that we would start seeking are:
- uncontrolled nested entities
- make basetype optional
- clarify or remove complex user cases: e.g. UNION and LIST