# simplify() does not work?

I enter:

```
sage: a, b = var('a, b'); simplify((a + b)^2 - a^2 - 2*a*b - b^2)
```

and I would expect 0 (i.e. zero) as a result from simplify(), but instead I get back unmodified:

```
(a + b)^2 - a^2 - 2*a*b - b^2
```

Am I missing anything? I just started using it and it looks very promissing...