static final class NotifyingStringBuffer implements CharSequence, Appendable {

  new StringBuilder builder;

  long changes;

  L onChange;

  public synchronized Appendable append(char c) {

    builder.append(c); ret change();

  }

  public synchronized Appendable append(CharSequence csq, int start, int end) {

    builder.append(csq, start, end); ret change();

  }

  public synchronized Appendable append(CharSequence csq) {

    builder.append(csq); ret change();

  }

  public synchronized void setLength(int i) {

    builder.setLength(i); change();

  }

  public synchronized void trimToSize() {

    builder.trimToSize();

  }

  // internal

  NotifyingStringBuffer change() {

    ++changes;

    pcallFAll(onChange);

    this;

  }

  synchronized toString { ret builder.toString(); }

  public synchronized char charAt(int index) { ret builder.charAt(index); }

  public synchronized int length() { ret builder.length(); }

  public synchronized CharSequence subSequence(int start, int end) {

    ret builder.subSequence(start, end);

  }

  synchronized S substring(int start) { ret builder.substring(start); }

  int capacity() { ret builder.capacity(); }

  public synchronized void onChange(Runnable r) {

    onChange = listCreateAndAdd(onChange, r);

  }

}