code: public boolean removeAll(Collection<?> c) {
          if (!(c instanceof RegularEnumSet))
              return super.removeAll(c);
  
          RegularEnumSet<?> es = (RegularEnumSet<?>)c;
          if (es.elementType != elementType)
              return false;
  
          long oldElements = elements;
          elements &= ~es.elements;
          return elements != oldElements;
      }
docs: Removes from this set all of its elements that are contained in
  the specified collection.
  
  @param c elements to be removed from this set
  @return <tt>true</tt> if this set changed as a result of the call
  @throws NullPointerException if the specified collection is null