The reverse engineering project described in this paper is aimed at documenting a 6.4 million lines of code COBOL/IMS/DB2 system for world-wide car leasing. The ultimate goal is to re-implement that system. The system was originally developed in the 1980s with less than 3 million code lines and has since evolved to its current size. It survived the year 2000 date change and the Euro conversion as well as several major company reorganizations to preserve the continuity of the leasing service. Finally, after 30 years of service it is planned to retire the system. However, the first two attempts to replace it, one by automatically converting it and the other by replacing it with a standard package ended in failure. It is now planned to rewrite the system based on a specification derived from the current code base. That specification includes among other documents a documentation of the processing rules. The extracted rules are intended to act as guide to those writing the new code.