Adjusting a binary tree to satisfy the heap property.

Source: The Art of Prolog

Program source code: trees_heap.pl

/*
	heapify(Tree,Heap) :-
	  The elements of the complete binary tree Tree have been adjusted
	  to form the binary tree Heap, which has the same shape as Tree 
	  and satisfies the heap property that the value of each parent node
	  is greater than or equal to the values of its children.
*/
 
heapify(void,void).
heapify(tree(X,L,R),Heap) :-
   heapify(L,HeapL), heapify(R,HeapR), adjust(X,HeapL,HeapR,Heap).
 
adjust(X,HeapL,HeapR,tree(X,HeapL,HeapR)) :-
    greater(X,HeapL), greater(X,HeapR).
adjust(X,tree(X1,L,R),HeapR,tree(X1,HeapL,HeapR)) :-
    X < X1, greater(X1,HeapR), adjust(X,L,R,HeapL).
adjust(X,HeapL,tree(X1,L,R),tree(X1,HeapL,HeapR)) :-
    X < X1, greater(X1,HeapL), adjust(X,L,R,HeapR).
 
    greater(X,void).
    greater(X,tree(X1,L,R)) :- X >= X1.
 
%   Program 3.28    Adjusting a binary tree to satisfy the heap property