This model contains two classes A and B with 
associations R1 and R2 between them. 
The operation swap_b swaps the two R1 and R2 links
for an A object. The expected state resulting from the
operation is asserted by a postcondition. 
The command file uses features introduced in 
USE 2.1.0: note the expressions in the insert and
delete commands, and the new let command.

SwapLinks.use:
two classes A (with one Operation) and B
two associations between A and B
one object of type A has to be connected with
exactly one object of type B for each association
pre- and postconditions for the operation

SwapLinks.cmd:
two objects of type B and one Object a of type A
are created
a is connected with the other objects 
with two links from the different associations
then the operation is called and the objects 
swap (let is used)
