code: public final UnicodeSet complement(CharSequence s) {
          checkFrozen();
          int cp = getSingleCP(s);
          if (cp < 0) {
              String s2 = s.toString();
              if (strings.contains(s2)) {
                  strings.remove(s2);
              } else {
                  strings.add(s2);
              }
              pat = null;
          } else {
              complement(cp, cp);
          }
          return this;
      }
docs: Complement the specified string in this set.
  The set will not contain the specified string once the call
  returns.
  <br><b>Warning: you cannot add an empty string ("") to a UnicodeSet.</b>
  @param s the string to complement
  @return this object, for chaining