Write a Prolog sorting routine that is guaranteed to take O(n log n) time in the worst case. (Hint: Try merge sort; a description can be found in almost any algorithms or data structures text.)...

Write a Prolog sorting routine that is guaranteed to take O(n log n) time in the worst case. (Hint: Try merge sort; a description can be found in almost any algorithms or data structures text.) Consider the following interaction with a Prolog interpreter:

What is going on here? Why does the interpreter fall into an infinite loop? Can you think of any circumstances (presumably not requiring output) in which a structure like this one would be useful? If not, can you suggest how a Prolog interpreter might implement checks to forbid its creation? How expensive would those checks be? Would the cost in your opinion be justified?