> I don't understand why the last two lines are there.

I thought those lines were needed to avoid an infinite loop in 
pathological cyclic cases. But on further thought you're right, they 
aren't needed. I installed that patch; thanks.

> shouldn't `equal' be symmetric?

Yes, on its domain. But circular lists are outside its domain, and the 
documentation doesn't promise any particular behavior on them. It's OK 
if (equal a b) signals an error and (equal b a) does not. It's even OK 
if (equal a b) signals an error and a later call (equal a b) with 
exactly the same (unchanged) arguments does not (because the stack 
happens to have more room the second time). We still have symmetry in 
the sense that (eq (equal a b) (equal b a)) always either returns t or 
signals an error; it never returns nil.

I installed the attached doc patch to try to make this a bit clearer.