code: @Override
      public List<Sync> buildConstraint(BtrPlaceTree t, List<BtrpOperand> args) {
          if (!checkConformance(t, args)) {
              return Collections.emptyList();
          }
  
          @SuppressWarnings("unchecked")
          List<VM> s = (List<VM>) params[0].transform(this, t, args.get(0));
          if (s == null) {
              return Collections.emptyList();
          }
  
          if (s.size() < 2) {
              t.ignoreError("Parameter '" + params[0].getName() + "' expects a list of at least 2 VMs");
              return Collections.emptyList();
          }
          return Collections.singletonList(new Sync(s));
      }
docs: Build a sync constraint.
  
  @param t    the current tree
  @param args must be 1 set of vms. The set must not be empty
  @return a constraint