static <A> A optPar(ThreadLocal<A> tl, A defaultValue) {
  A a = tl.get();
  if (a != null) {
    tl.set(null);
    ret a;
  }
  ret defaultValue;
}

static <A> A optPar(ThreadLocal<A> tl) {
  ret optPar(tl, null);
}

static <A> A optPar(O[] params, S name) {
  for (int i = 0; i+1 < l(params); i += 2)
    if (eq(params[i], name))
      ret params[i+1];
  null;
}