So the problem is that the semantic of methods (car, cdr) is not well defined: it is either "give me that member" or "give me that member and guarantee next tim

So the problem is that the semantic of methods (car, cdr) is not well defined: it is either "give me that member" or "give me that member and guarantee next time the result will be same". And the solution is to clearly separate those by adding more method names.