Eiffel and 'Design by Contract'

Tony Hoare famously wrote of the programming language ALGOL 60 that it was "so far ahead of its time  that it was not only an improvement on its predecessors but also on nearly all its successors".  In a way, the same could also be said of Eiffel, a pure object-oriented programming language designed by Bertrand Meyer in 1985.  The key to Eiffel's superiority over "nearly all of its successors" is the idea of 'design by contract' (DBC). This idea provides key insights into many areas of programming and programming language design, and Meyer has used it to produce clean solutions to problems which have given rise to unnecessarily complex messes in other languages. The best exposition of 'Design by Contract' remains Meyer's book 'Object Oriented Software Construction' (2nd Edition) but Eiffel Software (the company founded by Meyer) also have some nice Macromedia Flash introductions to Eiffel and Design by Contract at http://www.eiffel.com/developers/presentations/.