User blog comment:B1mb0w/J Function Sandpit J 4/@comment-1605058-20151014132917/@comment-27173506-20160205161325

Using the FS \(\zeta_{0}[n]=0, \zeta_{0}[n]=\varepsilon_{\zeta_{0}[n-1]}\) and the quite logical definition \((\omega \uparrow \uparrow \omega)[n]=\omega \uparrow \uparrow n\), we have f_w^^^2(2)=f_w^^w(2)=f_w^^2(2)=f_w^w(2)=f_zeta_0(2)=f_e_0(2)=f_w^w(2). This is not true for all FSes.