Be very, very careful.

Relying on the details of someone else's implementation is a very bad thing to do. Oh, lots of people do it. It works. But it leads to extra complexity, potential problems with future changes to the library, etc, etc, etc.

In general assume that anything not explicitly documented can and will change. Work with the interface whereever possible. Anything which pokes around in internals you have no business accessing should be regarded as probable future breakage. And remember that modular code is broken into components, each of which has a simple interface to the outside world. Keep your interfaces simple and clean, only use those interfaces. you will save a lot of headaches...