Contracts are attached to interfaces, not to specifications. So when you have abstract base class, it defines contracts, and implementing classes must adhere to these contracts - the can only strengthen it, not weaken it. 

This way the user code need pnly be aware of the specification, not the implementation. 

So method that _are_ overridden retain their contracts. 

This is precisely like with types, since types are contracts (and vice versa, in a way).

Elazar