(バージョン情報なし。おそらく SVN 版にしか存在しないでしょう)
A PriorityQueue is very similar to a Queue. Values are pushed into the queue with an assigned priority, and the value with the highest priority will always be at the front of the queue.
Implemented using a max heap.
"First in, first out" ordering is preserved for values with the same priority.
Iterating over a PriorityQueue is destructive, equivalent to successive pop operations until the queue is empty.