/**
 * Callback function for the 'webmention_mf2_rel_mapper' filter.
 *
 * @param mixed $rel_mapper 
 * @return mixed The filtered value.
 */
function my_webmention_mf2_rel_mapper_callback( $rel_mapper ) {
    // Your code here.
    return $rel_mapper;
}
add_filter( 'webmention_mf2_rel_mapper', 'my_webmention_mf2_rel_mapper_callback' );

apply_filters( 'webmention_mf2_rel_mapper', $rel_mapper )