private Expression notStatement( List<Expression> args, String caller ) {
      
      if ( args.size() != 1 ) {
         System.err.println("not expected exactly one argument and got " + args.size() );
         return new Void();
      }
      
      // returns the logical inverse
      if ( args.get(0).show(defSubst, caller).compareTo("#f") == 0 ) {
         return new Value("#t");
      }
      else {
         return new Value("#f");
      }
   }