docs: def filter_pages(pages, pagenum, pagename):
      """ Choices pages by pagenum and pagename """
      if pagenum:
          try:
              pages = [list(pages)[pagenum - 1]]
          except IndexError:
              raise IndexError('Invalid page number: %d' % pagenum)
  
      if pagename:
          pages = [page for page in pages if page.name == pagename]
          if pages == []:
              raise IndexError('Page not found: pagename=%s' % pagename)
  
      return pages