1. Your sketch sounds about right. The problem is probably sticking to the discipline of turning it into code.
2. The infinite loop is troubling -- but looking at your code is the wrong thing. The goal is to empower yourself so that you can do such things on your own w/o help from others. Have you thought thru why the algorithm should terminate (step 7 of the gen-rec design recipe)? If so, have you checked your program to make sure it adheres to your reasoning? 3. You wrote "m thinking that the goal is to define mc-solvable? and use it in mc-solution sort of like the backtracking algorithm for finding-route." That's about right. In a sense, the MC problem generates a graph and the algorithm searches the graph for feasible routes from the initial state xxx | <> | ooo | | to the final state: | <>| xxx | | ooo -- Matthias _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users