static <A> A popFirst_ping(L<A> l) {
  ping();
  ret popFirst(l);
}

static <A> A popFirst_ping(Collection<A> l) {
  ping();
  ret popFirst(l);
}

static <A> L<A> popFirst_ping(int n, L<A> l) {
  ping();
  ret popFirst(n, l);
}