> Hmmm. I also suspect that the performance of a play would constitute > a derived work
You can also "perform" a computer program, it's generally called _use_. The program's output may be derivative. Given that the GPL has language that is extremely un-restrictive regarding use, I doubt it could be applied to restricting front-ends that run across the exec() interface. Copyright law allows you to control "public performance" of your work, I think this is a separate concept from "use". You could use this control, for example, to control use of your program on a web server, where the output of the program is presented to clients. Thanks Bruce