### simplify() does not work?

I ~~enter:a, ~~enter:

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

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