void skip(char c) {
   if (s.charAt(index) == c) {
     index++;
     return;
   } else {
     abortPrefetch();
   }
 }
  String readSymbol() {
    StringBuffer sbuf = new StringBuffer();
    while (index < s.length()) {
      char c = s.charAt(index);
      if (sbuf.length() == 0) {
        if (!Character.isJavaIdentifierStart(c)) {
          abortPrefetch();
        }
      } else {
        if (!Character.isJavaIdentifierPart(c)) {
          return sbuf.toString();
        }
      }

      index++;
      sbuf.append(c);
    }
    if (sbuf.length() == 0) {
      abortPrefetch();
    }
    return sbuf.toString();
  }