The pairing-heaps.plt package provides a unit for creating and manipulating pairing heaps. To learn more about pairing heaps, see the Wikipedia entry on pairing-heaps. The heaps created by this package are "persistent", meaning that operations like "insert" create a new heap which contains all the elemnts of the old heap plus those which have been inserted. Persistent datastructures are particularly useful for multi-threaded programs because no operations can alter a pre-existing heap (which can therefore be shared between multiple threads).