User blog comment:Wythagoras/Catching function in normal ordinals/@comment-11227630-20131114015734

You wrote theta function wrongly. Normal theta function allows only countable ordinals (i.e. ordinals<Ω) in 2nd argument. θα function allows only ordinals<Ωα+1 in 2nd argument. e.g. θ(Ω,Ω) is not allowed and you must write θ1(Ω,Ω) instead.