code: public boolean containsPrefixOf(String s) {
          SortedSet<String> sub = headSet(s);
          // because redundant prefixes have been eliminated,
          // only a test against last item in headSet is necessary
          if (!sub.isEmpty() && s.startsWith((String)sub.last())) {
              return true; // prefix substring exists
          } // else: might still exist exactly (headSet does not contain boundary)
          return contains(s); // exact string exists, or no prefix is there
      }
docs: Test whether the given String is prefixed by one
  of this set's entries.
  
  @param s
  @return True if contains prefix.