grouping choices
What are the other properties of the traffic light systems that vary
across different jurisdictions? In Europe, for example, the lights are
usually mounted in front of the intersections, whereas, in North
America, they are mounted after intersections. We therefore have a
decision point mountLocation for which there are two
exclusive choices. We cannot model such choices using optional
cardinality because one choice must exclude the other and optional
clafers can be both present in an instance.
For such purposes, Clafer provides group cardinality
which restricts how many children of a given clafer can be instantiated.
Group cardinality is specified before clafer’s name and can be given by
an interval n..m or a keyword. In our case, we need the
exclusive or group 1..1 (alternatively, can be specified
using a keyword xor).
Let’s add the mountLocation to our model:
The clafer mountLocation itself is a mandatory property
of the system and therefore the group constraint xor will
always be enforced: every system will always have exactly one of
inFront or behind clafers. Having both of them
or neither is not allowed.
More information: Tutorial: Group Cardinalities
How many model instances are possible now?
Let’s use ClaferConfigurator to generate all instances.
As the number of model instances grows exponentially, viewing all of
them all at once is not useful. In configurator, you can filter the
results by selecting and eliminating clafers;
it filters out the model instances which don't have or
do have a given clafer, respectively.
- To only see instances which contain a given clafer, click the checkbox to the left of the clafer in the leftmost column.
- To only see instances which do not contain a given clafer, double click the corresponding checkbox.
- To restore the original view, click the
Resetbutton.
The view Constraints lists your selections.
Close the configurator and move to the page 
complex grouping and nesting.
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