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// define custom category template
add_filter('single_template', 'check_for_category_single_template');
function check_for_category_single_template( $t )
{
  foreach( (array) get_the_category() as $cat ) 
  { 
    if ( file_exists(TEMPLATEPATH . "/single-category-".$cat->slug.".php") ) return TEMPLATEPATH . "/single-category-".$cat->slug.".php"; 
    if($cat->parent)
    {
      $cat = get_the_category_by_ID( $cat->parent );
      if ( file_exists(TEMPLATEPATH . "/single-category-".$cat->slug.".php") ) return TEMPLATEPATH . "/single-category-".$cat->slug.".php";
    }
  } 
  return $t;
}