nl: html4_singlets is a tuple with 9 elements: strings 'br', 'col', 'link', 'base', 'img', 'param', 'area', 'hr' and 'input'.
code: html4_singlets = ( 'br' , 'col' , 'link' , 'base' , 'img' , 'param' , 'area' , 'hr' , 'input' )