When an operation from a class is dragged and dropped to an interface, Abstract
property is not checked in properties.

steps to reproduce:
- drag and drop an operation from a class to an interface
- select to check the checkbox on Abstract  
- uncheck Abstract 
The following 'Invalid Property' error message will appear:
"The operation of an interface  must be abstract.
The Abstract property for this operation  will be set back to True."
- click OK -> Abstract is checked. 

The confusion here is that one does not see Abstract checked when first
looking into the Properties sheet after dragged/dropped from a class.  
Only when one done a couple of check/uncheck steps that one is told "well, you
can't really uncheck Abstract, you know!"

Either the msg is not informative enough OR the inherited
operation is not properly incorporated into the Interface
after dragged/dropped.  If you forgot about the rule and
never did attempt to select Abstract for such operation,
what would have happened?  would it still be correct?