It is meant for Lisp programmers who want to debug or visualize their own algorithms. Instead of printing tracing information to the Lisp listener (which everybody normally does to understand what's going on), more readable graphical representations of the algorithm's internals are sent to the GTFL client page.
GTFL also comes with mechanisms for visualizing complex (hierarchical) data or control structures. It provides functions for drawing trees and for hiding complexity in elements that expand when the user clicks on them.
It's written by Martin Loetzsch and can be found at http://martin-loetzsch.de/gtfl
gui debugging development