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I'm not a lawyer, but I have dealt with the situation you are talking
about (from both sides). Do you have any particular rights? No, not if
you sign the agreement. Can they force you to sign the agreement as a
condition of employment, despite the fact that you are already employed
by them? There, you had better talk to a lawyer, not a tech writer.
Generally though, non-disclosure and similar agreements are binding and
are not superceded by any other laws (that I'm aware of). If your
company owns your work, then it is theirs, and not yours. They paid you
for it, and that was your contract. If they choose not to allow you to
keep a sample, that is their right, according to the contract entered
into between you. The interesting question, again, is whether they can
force you to enter into the contract in the first place, and how
hard-ass they intend to be about modifying the terms of the contract
just a bit.