private List<Queen> createQueenList(NQueens nQueens) {
   int n = nQueens.getN();
   List<Queen> queenList = new ArrayList<Queen>(n);
   long id = 0;
   for (Column column : nQueens.getColumnList()) {
     Queen queen = new Queen();
     queen.setId(id);
     id++;
     queen.setColumn(column);
     // Notice that we leave the PlanningVariable properties on null
     queenList.add(queen);
   }
   return queenList;
 }