array assign can nuke derivatives

Array assignment was incorrectly clearing the derivatives of all entries instead of just the derivatives of the entry being assigned.

[ckulla, 2010-02-09 21:40:51]

On 2010/02/08 19:54:50, ckulla wrote:
> 

Any comments on this?

[clifford.stein, 2010-02-09 21:55:56]

On 2010/02/09 21:40:51, ckulla wrote:
> On 2010/02/08 19:54:50, ckulla wrote:
> > 
> 
> Any comments on this?

LGTM

[aconty, 2010-02-09 21:57:23]

LGTM2