static final class BetterStringBuffer 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();
  }
  
  BetterStringBuffer change() {
    ++changes;
    pcallFAll(onChange);
    this;
  }
  
  toString { ret str(builder); }
}