code: public String popString() throws ParseException
      {
          final Object popped = super.pop();
          if (popped instanceof String)
              return (String) popped;
          /*
           * This is probably an unquoted single word literal.
           */
          if (popped instanceof TokVariable)
              return ((TokVariable) popped).getName();
  
          throw new ParseException("Literal required, found " + popped.getClass().getSimpleName(), 0);
      }
docs: <p>
  popString.
  </p>
  
  @return a {@link java.lang.String} object.
  @throws java.text.ParseException if any.