code: public String getAsPennTreebankString() {
          StringBuilder sb = new StringBuilder();
          sb.append("(");
          getAsPennTreebankString(1, 1, sb);
          sb.append(")");
          return sb.toString();
      }
docs: Gets a string representation of this parse that looks like the typical
  Penn Treebank style parse.
  
  Example:
  ((ROOT (S (NP (NN time))
  (VP (VBZ flies)
  (PP (IN like)
  (NP (DT an)
  (NN arrow)))))))
  
  @return A string representing this parse.