This example describes a graph with nodes and edges.
The operation
newTarget creates a new link (edge) between a
source node and exactly one new node.
This operation uses the operation oclIsNew.

The example corresponds to
the section `Validating Pre- and Postconditions
with USEŽ

Graph.use:
one class Node and one association Edge
one operation newTarget with two postconditions

Graph.cmd:
three calls of newTarget
both postconditions evaluate to true after the first call
the first postcondition evaluates to false and the
second to true after the second call
and the first postcondition evaluates to true
and the second to false after the last call