Consider the following interaction with a Prolog interpreter:

?- Y = X, X = foo(X).
Y = foo(foo(foo(foo(foo(foo(foo(foo(foo(foo(foo(
foo(foo(foo(foo(foo(foo(foo(foo(foo(foo(foo(foo(
foo(foo(foo(foo(foo(foo(foo(foo(foo(foo(foo(foo(
foo(foo(foo(foo(foo(foo(foo(foo(foo(foo(foo(foo(
foo(foo(foo(foo(foo(foo(...

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?