User blog comment:Boboris02/MBOT/@comment-24920136-20161219043958/@comment-24920136-20161219163312

I won't give you a definition of diagonalization but, what i mean by "cant employ diagonalization" is that we cant just assume that what comes next after a defined sequence of phi systems is also defined.

In order to have a f_6(n) equivalent for example he must manually enter "x system inside a y system inside a z...." and so on, so he can't actually reach f_w(n) with a finite amount of words.

The reason for this is that the  symbols aren't well defined unless you manually set them at each step.