Conditional checks for subpages

gistfile1.php

<?php
/**
 * Checks for sub pages
 *
 * This see if the current page is the page, or is a sub page 
 * of the page
 *
 * @since 1.0.0
 * @link http://codex.wordpress.org/Conditional_Tags#Testing_for_sub-Pages
 * @param int $pid, post id
 * @return boolean
 */
function is_tree( $pid ) {
    global $post;

    if ( is_page( $pid ) )
        return true;

    $anc = get_post_ancestors( $post->ID );
    foreach ( $anc as $ancestor ) {
        if ( is_page() && $ancestor == $pid ) {
            return true;
        }
    }

    return false;
}