Pre-condition: the object is empty. This ensures that the object does not contain any other objects.

Note that we unconditionally try to remove all objects. If a container needs to have some objects that can be removed and others that can't (such as components within the container), then the container will have to be implemented as a ComplexContainer - the non-removable components should be made contents of the enclosing ComplexContainer, and the secret inner container should be the one subject to this precondition.

objEmpty :   PreCondition