[plt-scheme] some more frtime questions

> Got it, thanks. After I fixed this though, it only worked for the first
> bullet, and returned errors for subsequent ones.
>
> If I remove the filter, it works fine - i.e:

What was the error it returned when you used the filter?  Depending on
how you've set things up, you may need to filter with (compose
sphere-struct? value-now) instead of just sphere-struct?.

In the mean time, you can keep the list from growing without bound by
replacing the call to filter with code that truncates the list to some
maximum length.

> (define all-bullets
>   (collect-b
>    bullet-e '()
>    (lambda (bullet lst)
>      (cons bullet lst))))
>
> I've also added some waves of enemies, not very well implemented yet,
> but a start:
> http://www.flickr.com/photos/7905273@N07/2233068235/

Looks like fun!  :-)

Cheers,
Greg