static class InterruptibleString implements CharSequence {
   S inner;

   *() {}
   *(S *inner) {}

   @Override
   public char charAt(int index) {
      ping();
      ret inner.charAt(index);
   }

   @Override
   public int length() {
      ret inner.length();
   }

   @Override
   public CharSequence subSequence(int start, int end) {
      ret new InterruptibleString(inner.substring(start, end));
   }

   @Override
   toString { ret inner.toString(); }
}