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.
On Thu, Oct 30, 2014 at 05:09:44AM -0700, Colin Macdonald wrote:
>    In other threads I think we agree that if not dif.has(Symbol) could change
>    to if dif.is_number (note lower case), but I've not done so here.

Yes, that's a prefereble way.  -1 for comments, +1 for clear code.