y look and feel to be &quot;incomplete.&quot; That
              is, you might not need to support the complete set
              of components.
            For example, an auxiliary look and feel might choose
	    to provide a <code>ButtonUI</code> subclass but no
              <code>LabelUI</code> subclass.
	     This
              option is allowed, and the multiplexing look and feel gracefully
              handles such situations.</p>
            <p>By default, however, Swing issues an error message wh