1. A method, comprising: adding a plurality of constraints to a plurality of queues having different priorities, each constraint of the plurality of constraints associated with a respective priority of a queue to which that constraint is added, the plurality of queues comprising a first queue and a second queue that has a lower priority than said first queue; adding a choice-constraint to either the first queue or the second queue based on a role of a user who submitted the particular choice; determining a conflict between the choice-constraint and a particular constraint in either said first queue or said second queue; and returning information about said conflict, said information identifying the particular constraint and a priority of the queue that holds said particular constraint, wherein the method is performed by one or more computers.