// 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 n1 = lCommonPrefix(a, s), n2 = lCommonPrefix(b, s);
  ifdef longestCommonPrefixOfNavigableSetAndString_debug
    printVars_str("longestCommonPrefixOfNavigableSetAndString", +s, +set, +n1, +n2);
  endifdef
  int n = Math.max(n1, n2);
  ret takeFirst_string(s, n);
}