code: public static List<Precedence> newPrecedence(Collection<VM> vmsBefore, VM vmAfter) {
          return newPrecedence(vmsBefore, Collections.singleton(vmAfter));
      }
docs: Instantiate discrete constraints to force a single VM to migrate after a set of VMs.
  
  @param vmsBefore the VMs to migrate before the other one {@see vmAfter}
  @param vmAfter   the (single) VM to migrate after the others {@see vmsBefore}
  @return the associated list of constraints