static <A> A addUnlessNull(Collection<A> l, A a) {
  if (l != null && a != null) l.add(a);
  ret a;
}

ifclass MultiSet
static <A> A addUnlessNull(MultiSet<A> ms, A a) {
  if (ms != null && a != null) ms.add(a);
  ret a;
}
endif