if you program in Lisp the function list-length gives nil for circular lists. Ninety-nine problems contains similar problems with lists. I should have said

if you program in Lisp the function list-length gives nil for circular lists. Ninety-nine problems contains similar problems with lists. I should have said that the difference between increments must be coprime with n to guarantee that the differece becomes 0 module n, that is both pointers get equal.