Description:

The Heap collection of modules provide routines that manage
a heap of elements. A heap is a partially sorted structure
that is always able to easily extract the smallest of the
elements in the structure (or the largest if a reversed compare
routine is provided).

If the collection of elements is changing dynamically, the
heap has less overhead than keeping the collection fully
sorted.

The elements must be objects as described in "Heap::Elem"
and all elements inserted into one heap must be mutually
compatible - either the same class exactly or else classes that
differ only in ways unrelated to the Heap::Elem interface.