func (pq *PriorityQueue) Update(item *Item, value string, priority int64) {
	item.value = value
	item.priority = priority
	heap.Fix(pq, item.index)
}
update modifies the priority and value of an Item in the queue.
package gtimer
import (
	"container/heap"
)
type PriorityQueue []*Item
func (pq *PriorityQueue) Len() int { return len(*pq) }
func (pq *PriorityQueue) Less(i, j int) bool {
	return (*pq)[i].priority < (*pq)[j].priority
}
func (pq *PriorityQueue) Swap(i, j int) {
	(*pq)[i], (*pq)[j] = (*pq)[j], (*pq)[i]
	(*pq)[i].index = i
	(*pq)[j].index = j
}
func (pq *PriorityQueue) Push(x interface{}) {
	n := len(*pq)
	item := x.(*Item)
	item.index = n
	*pq = append(*pq, item)
}
func (pq *PriorityQueue) Pop() interface{} {
	old := *pq
	n := len(old)
	item := old[n-1]
	item.index = -1  
	*pq = old[0 : n-1]
	return item
}