A potential future enhancement is to allow writing to any ResourceSet,
not just the main ResourceSet although that adds all sorts complications
including:
- which ResourceSet to write to
- unexpected behaviour when deleting a resource from one ResourceSet since
that may unmask a resource in a lower priority ResouceSet so what was a
delete looks like a replace with the user having no idea where the 'new'
resource came from
- how to handle PUT when the target is read-only but it could be written to
a higher priority ResourceSet that is read-write