Design by contract

Hi, I would like to implement Design by Contract for the base language. I have seen that exists a language (baselanguage.contracts) but it seems that I can add pre/post conditions only in interfaces (am I wrong?).

I would like instead to implement:

• pre/post constraints for interface and class methods with proper rules for inheritance
  

  - invariants

to do that I would need to change the generation rules also for methods which do not contains directly a pre-condition. For example in this case:

class A {
postcondition x
void foo();
}

class B extends A {
void foo();
}

while method foo in class B has no directly postconditions it should inherit postcondition x. So I would like to decorate the code normally generated for methods with the proper pre/post conditions check.

Which is the best way to do that?