/* compare two timers' target values, return the one with lower value. 
if either is NULL, return the other. if both are NULL, return NULL.  */
static xtimer_t* _compare(xtimer_t *a, xtimer_t *b)
/* compare two timers' target values, return the one with lower value. 
if either is NULL, return the other. if both are NULL, return NULL.  */
static xtimer_t* _compare(xtimer_t *a, xtimer_t *b)
{
    if (a && b) {
        return a->target <= b->target ? a : b;
    }
    else {
        return a ? a : b;
    }
}