data.priority-map implements priority maps in Clojure. A priority map is very
 similar to a sorted map, however its entries are sorted by value rather than
 by key.
 .
 In addition to supporting all the functions a sorted map supports, a priority
 map can also be thought of as a queue of [item priority] pairs. To support
 usage as a versatile priority queue, priority maps also support the
 conj/peek/pop operations