!752

static S prefix = "history";

answer {
  L list = getHistory();
  
  if (matchStart(prefix, s, m)) {
    s = m.rest();
    
    if "length"
      ret lstr(list);
    
    if "fields"
      ret structure(fields(list));
  }
}

static L<S> fields(L l) {
  new TreeSet<S> set;
  for (O o : l)
    if (o != null)
      set.addAll(objectToMap(o).keySet());
  ret list(set);
}