Earlier quoted context omitted.
>The distinguishing feature of Contract Programming is how it handles inheritance. How does it handle it?
It applies the Liskov substitution principle to it. So preconditions can only be widened in subclasses, and postconditions can only be narrowed. In simple terms, it means that when you override a method, the override's precondition is automatically OR'ed with that of the superclass, and your postcondition is AND'ed. And ditto for interface inheritance.
Thanks, that makes sense. IIRC, I read about this either in Bertrand Meyer's book Object-oriented Software Contruction, or in the Eiffel language's docs, some years back, though I may not have known about it as an application of the Liskov substitution principle at the time, or even later, although I did read about the Liskov principle later :)
And I may not have grokked the Liskov principle when I read it, either, but thanks to your reply, I now have a glimmering of what it means, and will look it up again :)