That shouldn't work at all. Both upgraph and downgraph are defined to take a single argument, but you are passing multiple...

That could be the problem. Make sure also that you aren't calling bitset with '().

Sorry I can't be of more help.

Ah, yes... I believe the word is "embarrassing"...
There are time when I feel rather blind and stupid. This is one of them.

I don't think I'd be calling bitset with '(), as a null list input would just return a null list. But that assumes I allowed it to take another argument....

Alright still having problems...

The same problem?

I see nothing wrong with your code... (I can't test it myself ATM, as I'm just recovering from a system wipe and have yet to reinstall scheme.)

Do you think it might be barfing because upgraph evaluates to '() ?

Yes, same problem.

I was thinking that may be the case... but if that's so, I'd really love to know why. It wouldn't make any sense: the empty list is just like any other piece of data.

I was also thinking it may have something to do with the (begin) statement. I'd elaborate more on this, but I've spent the last 7 hours learning LaTeX and using it to writing a lab report that's due tomorrow, and my brain is fried... Sleep time; I will return to this... probably Friday, as I have a lot to get done tomorrow.

A begin is implied in the body of a lambda.

The problem is that you have to be extra careful to keep data and code separate in scheme. You are mixing the two.

For example, the following is not a valid program:
(1 2 3 4)

but it is valid data:
'(1 2 3 4)

Essentially, your update function body at some point evaluates to
('() '())
...which is not a valid program.

Again, I have yet to re-install PLT so I can't be sure (it has been a few years since I've used scheme much), but you might fix it by simply changing your letrecs to use the following if inside:
(if (not (null? lst))

That way, the result is nothing, rather than an empty list.

Hope this helps.

Sorry, I'm not fully following you. I understand what you're saying about ('() '()) being unintelligible for the interpreter, but how do I rectify this?

Thanks again.

You are getting lost on syntactic sugar.
(define (foo x y z) (+ (- x y) z))
is sugar for:
(define foo (lambda (x y z) (+ (- x y) z)))

Likewise, inside the body of a lambda, using
(define (bar q) (* q (+ q 1))) ...
is sugar for:
(letrec ((bar (lambda (q) (* q (+ q 1))))) ...)

What I suggest is replacing the lines:
(if (null? lst) lst
with:
(if (not (null? lst))

and see if that works. (I'm not sure if it will or not...)

Rock!

Syntactic sugar causes cancer of the semi-colon.

I actually had that at one point, but my function call was embarrassing, so I had changed it, hoping it had something to do with my error.

Anyway, yeah... Now I need to remember to filter through the syntactic sugar when I'm stuck.

Thanks again,