static <A> L<A> concatLists_conservative(L<A> a, L<A> b) {
  if (empty(a)) ret b;
  if (empty(b)) ret a;
  ret concatLists(a, b);
}

static <A> L<A> concatLists_conservative(Collection<A> a, Collection<A> b) {
  if (empty(a) && b instanceof L) ret b/L;
  if (empty(b) && a instanceof L) ret a/L;
  ret concatLists(a, b);
}