Comment by gglon

Thanks, I forgot to remove M=Nest that was beneficial in my previous attempts where I used 3 Nests.

But remember:

Never forget that it is a waste of energy to do the same thing twice, and that if you know precisely what is to be done, you need not do it at all. --- E. E. ``Doc'' Smith (1930)

      ...the optimal solution avoids all pattern.
                                        --- Douglas Hofstadter (1983)

http://djm.cc/bignum-results.txt

So I would recommend to avoid things like f@f@f@f@a where there is clearly a pattern.