Called as the first operation as this object is being erased or unerased. 

This function is notified just before the current object is to be erased, giving this function the ability to cancel the erase. 

Returns eOk if and only if erase() is to continue. 

When overriding this function: 

1) If the OdDgElement's state is incorrect, return something other than eOk. 

2) If the parent class's subErase() returns anything other than eOk, immediately return it. 

3) If other actions are required before erase, do them. 

4) Return eOk. 

If you must make changes to this object's state, either make them after step 2, or roll them back if step 2 returns other than eOk. 

The default implementation of this function does nothing but return eOk. This function can be overridden in custom classes.