code: public boolean canShow() {
  		WizardPage<DATA> back = getPreviousPage();
  		while (back != null && back.isValid()) {
  			back = back.getPreviousPage();
  		}
  		return back == null;
  	}
docs: Returns whether this page can already be shown to the user. Default implementation checks whether all previous
  pages have been validated.
  
  @return true if page can be shown