code: public final UnicodeSet add(CharSequence s) {
          checkFrozen();
          int cp = getSingleCP(s);
          if (cp < 0) {
              strings.add(s.toString());
              pat = null;
          } else {
              add_unchecked(cp, cp);
          }
          return this;
      }
docs: Adds the specified multicharacter to this set if it is not already
  present.  If this set already contains the multicharacter,
  the call leaves this set unchanged.
  Thus "ch" =&gt; {"ch"}
  <br><b>Warning: you cannot add an empty string ("") to a UnicodeSet.</b>
  @param s the source string
  @return this object, for chaining