How can a script tell Prism to quit?

There is no script command to make Prism quit. Instead, Prism follows this rule. When the script ends: If you launched Prism from another program AND if all files are closed when the script ends THEN Prism itself will close too. So there is no need for a command to exit Prism. You simply have to use the CLOSE command in the script to close all open files, and then Prism will quit itself when the script is done.