/* merge two timer lists, return head of new list  */
static xtimer_t* _merge_lists(xtimer_t *head_a, xtimer_t *head_b)
/* merge two timer lists, return head of new list  */
static xtimer_t* _merge_lists(xtimer_t *head_a, xtimer_t *head_b)
{
    xtimer_t *result_head = _compare(head_a, head_b);
    xtimer_t *pos = result_head;
    while(1) {
        head_a = head_a->next;
        head_b = head_b->next;
        if (!head_a) {
            pos->next = head_b;
            break;
        }
        if (!head_b) {
            pos->next = head_a;
            break;
        }
        pos->next = _compare(head_a, head_b);
        pos = pos->next;
    }
    return result_head;
}