code: public boolean addAll(Collection<? extends E> c) {
          if (!(c instanceof RegularEnumSet))
              return super.addAll(c);
  
          RegularEnumSet<?> es = (RegularEnumSet<?>)c;
          if (es.elementType != elementType) {
              if (es.isEmpty())
                  return false;
              else
                  throw new ClassCastException(
                      es.elementType + " != " + elementType);
          }
  
          long oldElements = elements;
          elements |= es.elements;
          return elements != oldElements;
      }
docs: Adds all of the elements in the specified collection to this set.
  
  @param c collection whose elements are to be added to this set
  @return <tt>true</tt> if this set changed as a result of the call
  @throws NullPointerException if the specified collection or any
  of its elements are null