public XElement addAttribute( XAttribute attribute ){
        if( attribute == null )
            throw new NullPointerException( "attribute must not be null" );
        
        if( getAttribute( attribute.getName() ) != null ){
        	throw new IllegalArgumentException( "attribute '" + attribute.getName() + "' has already been added to this element" );
        }
        
        attributes.add( attribute );
        return this;
    }