def ff m, *a
    return ->x{x+1} if m==0
    return ->f{->x{x.times{x=f[x]};x}} if m==1
    ->x{
        if a.size==m
            y=a[0], z=a[1]
            x.times{z=y[z]}
            a[2..-1].inject(z){|y,z|y[z]}[x]
        else
            ff m, [*a, x]
        end
    }
end


(ff 3)[ff 2][ff 1][ff 0][2]
= ff(3, ff(2), ff(1), ff(0))[2]
= (ff 2)[ff(2)[ff 1]][ff 0][2]
= ff(2, ff(2)[ff 1], ff(0))[2]
= ff(2)[ff 1][ff(2, ff(1), ff(0))][2]