function previous_post_link($format='« %link', $link='%title', $in_same_cat = false, $excluded_categories = '') {
adjacent_post_link($format, $link, $in_same_cat, $excluded_categories, true);
}
function next_post_link($format='%link »', $link='%title', $in_same_cat = false, $excluded_categories = '') {
adjacent_post_link($format, $link, $in_same_cat, $excluded_categories, false);
}
function adjacent_post_link($format, $link, $in_same_cat = false, $excluded_categories = '', $previous = true) {
if ( $previous && is_attachment() )
$post = & get_post($GLOBALS['post']->post_parent);
else
$post = get_adjacent_post($in_same_cat, $excluded_categories, $previous);
if ( !$post )
return;
$title = $post->post_title;
if ( empty($post->post_title) )
$title = $previous ? __('Previous Post') : __('Next Post');
$title = apply_filters('the_title', $title, $post->ID);
$date = mysql2date(get_option('date_format'), $post->post_date);
$rel = $previous ? 'prev' : 'next';
$string = '<a href="'.get_permalink($post).'" rel="'.$rel.'">';
$link = str_replace('%title', $title, $link);
$link = str_replace('%date', $date, $link);
$link = $string . $link . '</a>';
$format = str_replace('%link', $link, $format);
$adjacent = $previous ? 'previous' : 'next';
echo apply_filters( "{$adjacent}_post_link", $format, $link );
}
given the above code from link-template.php, how to use filter to add class on A tag?