Given a particular formula like a*(b+c*(d+e*f)), there aren't many "trivially equivalent" permutations. e and f can be swapped in all cases giving 6! / 2 instead of 6! permutations. There's so many other formulas with 6 terms though that one needs to cut down the possibilities immensely for this method to yield the result. One can swap the additions as well though that gives a different formula and makes counting truly distinct expressions more difficult. Actually this does give rise to an interesting sub-problem that I've made another thread about.