Relational: comments for part of the simplify code

I found this bit of code that uses `.equals(0)` confusing.  I think
I understand it now, maybe these comments would be useful for others.
> In fact, most likely there is a bug somewhere.

Maybe, but that doesn't necessarily correspond to a bug in this code.  We only use the .evalf if we *know* that dif is non-zero (when presumably we can guarantee that evalf will give the correct sign.  (the evalf is only used for inequalities).

I was worried about bugs of this type but IIRC, if we can trust `.equals(0)` to tell us the truth, then this code is robust.  I have not verified that `.equals(0)` does tell the truth ;-)