listing pool members of a given pool

I'm afraid this is a bit of a brick wall, and you are not the first to run into it. Each Tcl script in the presentation runs in its own interpreter, and completes before the page is rendered. There is no method for creating dependencies or sending data from one script to the next, and no interative execution that would run the script after your first selection. For example, assuming that you want to choose a pool in your first drop down, APL provides no way to send that pool name to the code that would generate a second drop down, and no way to signal that code to execute again. This architectural challenge has been submitted to the development team, and we hope to see a solution in a future release. Until then, I have a collection of what I think are the most creative uses of the APL "optional"--feel free to contribute. :-)