RulerColumnDescriptor desc= (RulerColumnDescriptor) array[i];
dag.addVertex(desc);
Set before= desc.getPlacement().getConstraints();
for (Iterator it= before.iterator(); it.hasNext();) {
RulerColumnPlacementConstraint constraint= (RulerColumnPlacementConstraint) it.next();
String id= constraint.getId();
RulerColumnDescriptor target= (RulerColumnDescriptor) descriptorsById.get(id);
if (target == null) {
noteUnknownTarget(desc, id);
} else {
boolean success;
if (constraint.isBefore())
success= dag.addEdge(desc, target);
else
success= dag.addEdge(target, desc);
if (!success)
noteCycle(desc, target);