EiffelWeasel, also known as eweasel, is a tool originally written by David Hollenberg from MOSIS (a division of the University of Southern California's Information Sciences Institute) in the early 1990's when the first Eiffel 3 compiler was released. The tool's goal was to ensure conformance of the ISE Eiffel compiler to the recently published specification and to detect any unforeseen regression from version to version. In 2006, USC agreed to open-source EiffelWeasel and it is now hosted as part of the EiffelStudio project repository.
You can read more about eweasel at:
To get started with eweasel read the Eweasel How To page.