User blog comment:SuperJedi224/An array notation/@comment-5029411-20140121231905

Here an extension!

[a(1)b] = [a^a^(b-1)]

[a,x(1)b] = [a^x,a^x...a^x,a^x]

[a(1)b,c] = [(a^a^(b-1))^c]

[a(1)b(1)c] = [(a^a^(b-1))^(a^a^(b-1))^(c-1)]

[a(2)b] = [a(1)a...a(1)a] (b a's)

[a(3)b] = [a(2)a...a(2)a] (b a's)

[a(c,d)b] = [a(c^d)b]

Limit is [a(a(...)a)a]