Works well on my 404 page

I included in my child theme 404.php file with an if function as follows:

<?php if ( function_exists('the_related_for_string') ) { // show links to possibly related content ?>
<p>Perhaps you were looking for one of our _ posts:</p>
<?php the_related_for_string( 'my site relevant keyword phrase' ); ?>
<?php } // end if ?>

Works like a charm ??