def page page
      if page_exists?(page)
        return BasicPage.new(self, normalize_page(page))

      else
        if null_page
          return null_page_instance
        else
          return nil
        end

      end

    end
def page page
      if page_exists?(page)
        return BasicPage.new(self, normalize_page(page))

      else
        if null_page
          return null_page_instance
        else
          return nil
        end

      end

    end
create page instance by page number.
 if page number you specified was not existed,
 nil would be returned. note, page start at 1, not zero.