code: public final UnicodeSet retain(CharSequence cs) {
  
          int cp = getSingleCP(cs);
          if (cp < 0) {
              String s = cs.toString();
              boolean isIn = strings.contains(s);
              if (isIn && size() == 1) {
                  return this;
              }
              clear();
              strings.add(s);
              pat = null;
          } else {
              retain(cp, cp);
          }
          return this;
      }
docs: Retain the specified string in this set if it is present.
  Upon return this set will be empty if it did not contain s, or
  will only contain s if it did contain s.
  @param cs the string to be retained
  @return this object, for chaining