a more efficient loopForestSteensgaard

[Stephen Weeks]

> The space requirements of this seem a bit scary.  

I'm not sure if you're up to the latest email, but I now believe that
the loopForest is linear space, and folding over all the elements in a
loop is also linear time and space.

> Isn't it just a matter of using the fold function as a
> representation of the `list'?  I.e., you never do the concatenate
> because all you would do with the result was some kind of fold over
> it.

Pretty much.