So you had the atomic form factor generalized to the case where the atom makes a transition from its ground state to an excited state, and you get that generalized form factor and then you multiply it by the initial and final wave function of the atom. And then you have a very much simpler theory. So this was the basis of my paper and having simplified it to that extent it was easy to do the integration, and it was easy then to do more than the... to sum the probability of various... excitations of the atom, to sum over all possible final states of the atom, and get the total cross section or total inelastic cross section of the atom, which comes out very easily. And then I found - and that was very nice - that it became even easier if you took not just the cross section of going from state zero to state n, but multiplied it by the energy difference, energy of state n minus energy of state zero. And if you did that you got a still simpler result, in fact I later found out - much later - that this really was the way in which Kuhn and Reiche had discovered their sum rule for atomic oscillator strengths. And this had been the basis of Heisenberg's Matrix Mechanics, (p*q)-(q*p); that's essentially what I rediscovered, and so I could sum the cross section multiplied by the energy change, and that of course is the energy loss of the integral particle. And so thereby I could calculate in a very simple way the energy loss of any particle going... interacting by electric forces with an atom.