code: public String startSymbol() {
      String[] ssyms = startSymbols();
      if (ssyms == null || ssyms.length == 0) {
        return null;
      }
      return ssyms[0];
    }
docs: Returns a String which is the first (perhaps unique) start symbol
  of the treebank, or null if none is defined.
  
  @return The start symbol