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<?php
class UL_Submenu_Walker extends Walker_Nav_Menu {
    private $hidden = false;

    function start_lvl(&$output, $depth) {
        $style = "";
        if($depth == 0) $style = (($this->hidden)?"":"display:none;");
        $output .= "<ul class=\"submenu-".$depth."\" style='".$style."'>";
    }

    function start_el(&$output, $item, $depth, $args) {
        $class_names = $value = '';

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;

        if($depth == 0 && (
            in_array("current-menu-item", $classes)
                || in_array('current-menu-parent', $classes)
                || in_array('current-menu-ancestor', $classes)) ) $this->hidden = true;
        else $this->hidden = false;
        parent::start_el($output, $item, $depth, $args);
    }
}