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);
  }
}