static <A> A popFirst(L<A> l) {
  if (empty(l)) null;
  A a = first(l);
  l.remove(0);
  ret a;
}

static <A> A popFirst(Collection<A> l) {
  if (empty(l)) null;
  A a = first(l);
  l.remove(a);
  ret a;
}

static <A> L<A> popFirst(int n, L<A> l) {
  L<A> part = cloneSubList(l, 0, n);
  removeSubList(l, 0, n);
  ret part;
}