It is possible with computers to put different errors into each copy
published and keep a record of (errors, user-copy-sent-to) pairings.
> Using the technique of injecting error into a document to identify its
> origin is only as useful as the limitations of the tools we use to
> manipulate it. Many of the tools we currently have empower the user to
> make their own presentation decisions -- discarding multiple spaces within
> a paragraph, for instance. While this has not yet been used to change
> the data once it has been retrieved -- perhaps for efficiency sake -- I
> would not consider it completely out of the question.
Yes, I agree for text it is hard (perhaps, as you say, impossible) to
put a hidden, non-removable watermark.
My concern is to add something that would be good enough to catch the
dumb copier -- who I claim will be the majority of copiers. I'm not
looking for something ironclad, rather something that would let unwary
users know that they have violated the rules. I have enough faith
left to believe that many people will want to follow the rules if the
rules are brought to their attention, the procedures for conpliance
are really easy, and the cost of documents is low.
(Why are we working on a Sunday?)