open source clause for employment contract?

[David Rosenstrauch]

I'm looking at one of those employment contracts that are increasingly
popular these days, where the employer claims ownership of any
"inventions" you create.  I participate in a lot of open source
projects, though, and would like to add some wording to the contract to
make sure that my ability to work on open source is protected by doing
things like exempting my open source work from employer ownership
(assuming of course that I don't use any of their proprietary code,
don't work on something that competes with one of their products, etc.)

Anyone know if there's any standard legalese floating around the FOSS
community that I could use as a starting point for this?  (My lawyer's
reasonably tech-savvy, but not totally up on FOSS issues, so I'd like to
give him a something to start from.)  I'm sure I can't be the only one
who's needed something like this.

Thanks,

DR