@Override
 public Object visit(final PlusNode nodeOnVisit) {
   final Object lhsNumVal = nodeOnVisit.left().data().obj();
   Object rhsNumVal = 0.0d;
   if (nodeOnVisit.right() instanceof PlusNode) {
     rhsNumVal = nodeOnVisit.right().visit(this);
   } else {
     rhsNumVal = nodeOnVisit.right().data().obj();
   }
   return (Double) lhsNumVal + (Double) rhsNumVal;
 }