Just for fun: Countdown numbers game solver

[Paul Rubin]

Terry Jones <terry at jon.es> writes:
> Here's a solution that doesn't use any copying of lists in for recursion.
> It also eliminates a bunch of trivially equivalent solutions. The countdown
> function is 37 lines of non-comment code.  Sample (RPN) output below.

Nice, I realized after I posted my 2nd solution that it was missing
some cases and it's good to see confirmation that 239 is reachable.
I'll see if I can fix my version.  I think the list copying is ok
given that the recursion depth always must be fairly small if the
generator is to complete in any reasonable amount of time.