So, insert the definition into its proper place: {lo broda cu brode} = {ze'a noi broda cu brode}. This latter is a tautology, since {ze'a} always refers to a thing which makes the predication true.
Finally, a practical note. Whatever the point of all this is, dealing with MEX now is bad idea. Given that the creators of Lojban don't know how MEX works and, to a great extent, don't even know how it should work, trying to explain it now is a thankless task foreddomed to failure. Eventually, when you have a successful theory (of whatever sort you are working on) for the core of Lojban, you will be in a position to apply that theory to MEX to explain how parts of it work and to criticize and guide the development of other parts. Hopefully. But not now and not yet as a part of e\developing that theory.