// NotifyingList is our new go-to way to wrap a list
// with additional functions.
persistable sclass SSIList extends NotifyingList<SSI> is ByteIO {
  *(Cl<SSI> l) { initAndAddAll(l); }
  
  public void readWrite(ByteHead head) {
    head.exchangeInt(-> size(), size -> addAll(repF(size, -> new SSI)));
    for i over this:
      head.exchange(data.get(i));
  }
}