Hmm.  I guess we just assume it is obvious, given that the with statement examples name the variable 'stack', and the pop_all docs mention that no callbacks are called until it is closed "or at the end of a with statement".  So, technically, using the result of a pop_all in a with statement is documented.  I could be made clearer though if someone wants to propose a doc patch.  I'm not sure it is worth expanding the example, though.