// return longest common prefix of s an an entry of set
static <A> L<A> longestCommonPrefixOfNavigableSetAndList(L<A> s, NavigableSet<L<A>> set, Comparator<A> comparator) {
  if (set == null || s == null) null;
  L<A> a = set.floor(s), b = set.higher(s);
  int n1 = lCommonPrefix_lists_comparator(a, s, comparator);
  int n2 = lCommonPrefix_lists_comparator(b, s, comparator);
  int n = max(n1, n2);
  ret takeFirst(s, n);
}