In the case of the grapplers, the set of grapplers is a composite object containing them. The constraint is set up from the grappled picture to the set of grapplers say, in effect: if the picture is moved, propagate a move operation to the grappler composite.
This section explores constraints, showing how they can be used to keep pictures aligned, and (more interestingly for many visual languages) to keep arrowed lines connected to pictures.