static S renderUserThreadsWithStackTraces() {
  new StringBuilder buf;
  printTo(buf, asciiHeading2("Threads"));
  Map<Thread, StackTraceElement[]> threadMap = Thread.getAllStackTraces();
  int n = 0;
  for (Thread t : threadMap.keySet()) {
    ThreadGroup g = t.getThreadGroup();
    if (g != null && g.getName().equals("system")) continue;
    ++n;
    printTo(buf, t);
    for (StackTraceElement e : threadMap.get(t)) {
      printTo(buf, "  " + e);
    }
    printTo(buf);
  }
  printTo(buf, n + " user threads.");
  ret str(buf);
}