private boolean checkBinary() {
   boolean isBi = true;
   for (Variable v : sys.getVars()) {
     Vector<Instance> vinst = v.getInstances();
     if (vinst.size() != 2) {
       isBi = false;
       break;
     }
   }
   sys.setIsBinary(isBi);
   return isBi;
 }