Isn't this a logical impossibility? To have knowledge is to contain it in your source code, so A is contained in B, and B is contained in A...

No. If you know all relevant data yourself you don't have to know it again just because B knows it. That is just a naive, inefficient way to implement the 'source code'. Call the code 'DRY' for example. Or consider it an instruction to do a 'shallow copy' and a 'memory free' after a getting a positive result for a 'deep compare'.