@Override
 public SEXP getVariable(Symbol name) {
   int index = list.getIndexByName(name.getPrintName());
   if (index == -1) {
     return Symbol.UNBOUND_VALUE;
   } else {
     return list.getElementAsSEXP(index);
   }
 }
 @Override
 public boolean hasVariable(Symbol name) {
   return list.getIndexByName(name.getPrintName()) != -1;
 }