User blog comment:Scorcher007/Large countable ordinal notation up to Z2 and ZFC/@comment-35470197-20181121071653/@comment-31580368-20181121112130

In S[σ](α), α - means any ordinal up to 1st fixed point of α↦S[σ](α); α↦S[σ](α) = S[σ](1undefined0)