code: public boolean containsAll(Collection<?> c) {
          if (!(c instanceof RegularEnumSet))
              return super.containsAll(c);
  
          RegularEnumSet<?> es = (RegularEnumSet<?>)c;
          if (es.elementType != elementType)
              return es.isEmpty();
  
          return (es.elements & ~elements) == 0;
      }
docs: Returns <tt>true</tt> if this set contains all of the elements
  in the specified collection.
  
  @param c collection to be checked for containment in this set
  @return <tt>true</tt> if this set contains all of the elements
  in the specified collection
  @throws NullPointerException if the specified collection is null