For storable purposes, it makes sense that some attributes of objects are not stored to disk. We call them transient attribute and currently are specified using a note clause:
field: detachable X note option: transient attribute end
An attribute can be marked `transient' if and only if:
- its type has a default value (i.e. it cannot be a formal or an attached type)
- it is not a user defined expanded type
- 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 transient 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.
The transient property of an attribute declared in a class A keeps its transient property in descendants of A unless the attribute is redefined.