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//Add search to end of theme
add_filter( 'wp_nav_menu','add_search_box', 10, 2 );
function add_search_box( $items, $args ) {
    if ( 'primary_navigation' == $args->theme_location )
    $items .= '<li class="search">' . get_search_form( false ) . '</li>';
    return $items;
}