我使用下划线。我来做一个项目。我想将特色图片（缩略图）添加到上一篇/下一篇文章的链接中。他们有一个我正在编辑的函数叫做THEMENAME_post_nav(). 我正在努力调整此函数，使其包含缩略图。有人能帮忙吗？代码如下：

function THEMENAME_post_nav() {
// Don\'t print empty markup if there\'s nowhere to navigate.
$previous = ( is_attachment() ) ? get_post( get_post()->post_parent ) : get_adjacent_post( false, \'\', true );
$next     = get_adjacent_post( false, \'\', false );

// This is how I\'m trying to include the thumbnails
$prevthumbnail = get_the_post_thumbnail($previous->ID);
$nextthumbnail = get_the_post_thumbnail($next->ID);

if ( ! $next && ! $previous ) {
    return;
}
?>
<nav class="navigation post-navigation" role="navigation">
    <h1 class="screen-reader-text"><?php _e( \'Post navigation\', \'THEMENAME\' ); ?></h1>
    <div class="nav-links">
        <?php

            previous_post_link( \'<div class="nav-previous">%link</div>$prevthumbnail\', _x( \'<span class="meta-nav">&larr;</span> %title\', \'Previous post link\', \'THEMENAME\' ) );
            next_post_link( \'<div class="nav-next">%link</div>$nextthumbnail\', _x( \'%title <span class="meta-nav">&rarr;</span>\', \'Next post link\', \'THEMENAME\' ) );
        ?>
    </div><!-- .nav-links -->
</nav><!-- .navigation -->
<?php
}