public static function comboSetSelectionsAreValid($comboSetId, $grouptoitemmap) {
        $comboSetId = (int)$comboSetId;
        $comboset = ComboSet::getComboSetById($comboSetId);
        if ($comboset instanceof GenericError) {
            return $comboset;
        }

        $grouptoitemmap = Cart::sanitizeComboSetSelections($grouptoitemmap);

        // check that all required groups have selections
        foreach ($comboset->getGroups() as $group) {
            if ($group->getOptional() === false) {
                // if the required group has at least one item but no item is selected, abort
                if (count($group->getItems()) > 0 && empty($grouptoitemmap[$group->getId()])) {
                    return Master::getErrorStore()->getErrorResponse(
                        ErrorStore::COMBO_GROUP_IS_REQUIRED,
                        [$group],
                        [$group],
                        [$group]
                    );
                }
            }
        }

        // check that all selected groups and items exist
        foreach ($grouptoitemmap as $groupid => $itemids) {
            $group = ComboGroup::getComboGroupById($groupid);
            // if the group does not exist OR it is not associated with the combo-set, abort
            if ($group === null || $group->getComboSetId() !== $comboset->getId()) {
                return Master::getErrorStore()->getErrorResponse(
                    ErrorStore::COMBO_GROUP_NOT_FOUND,
                    [$groupid],
                    [],
                    [$groupid]
                );
            }

            foreach ($itemids as $itemid) {
                $item = GroupItem::getGroupItemById($itemid);
                // if the item does not exist OR it is not associated with the group, abort
                if ($item === null || $item->getGroupId() !== $group->getId()) {
                    return Master::getErrorStore()->getErrorResponse(
                        ErrorStore::GROUP_ITEM_NOT_FOUND,
                        [$itemid],
                        [],
                        [$itemid]
                    );
                }
            }
        }

        return true;
    }