This is a binary heap based priority queue implementation. In addition to the properties and operations common to all binary heaps (see http://en.wikipedia.org/wiki/Binary_heap), this implementation also supports efficient find, update, replace, and delete operations. Please see the docstrings of the exported symbols for more information, especially make-empty-queue for an explanation of how item lookups are achieved. -------------------------------- Acknowledgments This code was originally based off of Peter Norvig's implementation found here: http://aima.cs.berkeley.edu/lisp/utilities/queue.lisp Norvig's code was built on top of the heap algorthms in "Introduction to Algorithms" by Corman, Leiserson, and Rivest. I trivially updated some of that code to match the 2nd edition of the book by Corman, Leiserson, Rivest, and Stein. Specifically, I updated the referenced page numbers in the docstrings and implemented the Heap-Increase-Key algorithm (called improve-key in my code), which was not in the first edition (and is very useful for the new operations I added).