I was trying to understand the structure of (a-b), where "a" and "b" were two very large expressions. So I was doing things like (a-b).simplify_full().factor(). I discovered the following anomaly.

```
sage: var ('a', 'b') #Adequate for illustration
(a, b)
sage: (a-b); (a-b).factor()
a - b
a - b
sage: b=a #By chance, "a" and "b" were equivalent.
sage: (a-b); (a-b).factor()
0
Traceback (most recent call last):
...
ArithmeticError: factorization of 0 is not defined
```

I would have expected 0.factor() to give 0.