This Example shows the use of openter and opexit
for an operation with side effects. 
With four postconditions different expressions using 
@pre are presented.

AB.use:
two classes A,B and one association R

AB.cmd:
creates state 1 with two objects and one link
calls the operation with openter
creates state 2 by chancing attribute values,
changing links and creating a new object
then opexit

AB1.cmd:
creates state 1

AB2.cmd:
creates state 2

