Pitfalls and shortcomings are still to be coped with, however, both on the more fundamental issues (e.g., the nature of the E(N) function and the issue of differentiability, the convergence of the E = E[N, v] perturbation series in N and v) and on more applied aspects (e.g., the delineation of the scope of the various principles) before the next step, from interpretation to prediction, can safely be taken; see Section 4.8.