User blog comment:Googleaarex/BMS encoded to 1 symbol system/@comment-30754445-20170720221437/@comment-30754445-20170721004036

Yeah, 51512 is the best I got as well. And it certainly looks like the winner to me.

Expanding it abit more will get us:

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

Then:

(0)(1,1)(2,2)(3,2)(4,2)(5,2)(6,2)[4]

Which, unless I'm mistaken, correspond to ψ₀(ψ₁(ψ₁(ψ₁(ψ₁(0))))