code: public UnicodeSet retainAll(UnicodeSet c) {
          checkFrozen();
          retain(c.list, c.len, 0);
          strings.retainAll(c.strings);
          return this;
      }
docs: Retains only the elements in this set that are contained in the
  specified set.  In other words, removes from this set all of
  its elements that are not contained in the specified set.  This
  operation effectively modifies this set so that its value is
  the <i>intersection</i> of the two sets.
  
  @param c set that defines which elements this set will retain.