code: public void add(PageFactory<? extends IRequestablePage> pageSource)
          throws IllegalArgumentException {
          validateNotNull(pageSource, "pageSource");
          Class<? extends IRequestablePage> pageClass = pageSource.getPageClass();
          validateNotNull(pageSource, "pageClass");
          synchronized (this) {
              contents.put(pageClass, pageSource);
          }
      }
docs: <p>add.</p>
  
  @param pageSource a {@link org.ops4j.pax.wicket.api.PageFactory} object.
  @throws java.lang.IllegalArgumentException if any.