Transient Attributes

For storable purposes, it makes sense that some attributes of objects are not stored to disk. We call them volatile attribute and currently are specified using a note clause:

field: detachable X
   note
     option: volatile
   attribute
   end

Validity Rule

An attribute can be marked `volatile' if and only if:

1. its type has a default value
2. it is not a user defined expanded type
3. the enclosing class is not expanded

Rationale of validity rule

If the type did not have a default value (which is the case for attached types), then upon retrieval the volatile attribute would be Void which is against void-safety.

For simplicity and backward compatibility of `basic_store' which does a memory copy of the object to disk, it would be quite complicated to implement when expanded are involved. We prefer our users to use SED once it supports expanded.