LOGEN is an offline partial evaluation system for Prolog written using the so called ”cogen approach”. Basically, the cogen is a system which: 1. based upon an annotated version of the program to be specialised produces a specialised partial evaluator for that program. This partial evaluator is called a generating extension. 2. the generating extension can be used to specialise the program in a very efficient manner. Try out Logen without having to install it on your machine by clicking on the picture above.