The listener is the program that the user interacts with to execute other programs. It is similar to the Xterm program for the X window system.
We can probably use the listener written by Andy Hefner, and that is included in the McCLIM distribution. Andy says that it is a very simple program, so adapting it to our needs should not be too hard.
The listener should allow the user to type arbitrary Lisp expressions, but, more importantly, it should have shortcuts for starting application programs (possibly in their own threads), and for managing the