// return longest common prefix of s an an entry of set
sS longestCommonPrefixOfNavigableSetAndString(S s, NavigableSet<S> set) {
  if (set == null || s == null) null;
  S a = set.floor(s), b = set.higher(s);
  int n = max(lCommonPrefix(a, s), lCommonPrefix(b, s));
  ret takeFirst(s, n);
}