static <A> L<A> addAllAndReturnNew(Collection<A> c, Collection<A> b) {
  if (c == null || b == null) ret emptyList();
  new L<A> newEntries;
  for (A a : b)
    if (c.add(a))
      newEntries.add(a);
  ret newEntries;
}

Began life as a copy of #1015243