public function addSubMenuItem(string $k, string $v = '', string $prefix = P)
    {
        // Check if may be added if disables
        if (!$this->isAddingItemsAllowed()) {
            return $this;
        }

        // set value as key if not provided
        if (!$v) {
            $v = $k;
        }

        // Check that menu for this submenu item exists
        if (!isset($this->_menu[$prefix])) {
            return $this;
        }

        // Check if submenu item already exists
        if (isset($this->_menu[$prefix][$k])) {
            return $this;
        }

        // Check if have access to main menu item
        if (!Users::getInstance()->checkAccess($prefix, $k)) {
            return $this;
        }

        // Make main menu as array to add submenu items
        if (!is_array($this->_menu[$prefix])) {
            $this->_menu[$prefix] = [
                $prefix => $this->_menu[$prefix],
            ];
        }

        // Add submenu item
        $this->_menu[$prefix][$k] = $v;

        return $this;
    }