User blog comment:Vel!/FGH Gripe/@comment-2033667-20150326185550/@comment-1605058-20150327191845

@Vel! This paper of Weiermann actually discusses FGH up to ordinals of the form $$\psi_0\Omega_{..._{\Omega_\omega}}$$, but I'm sure he explicitly defines FS's for all ordinals used.