User blog comment:SuperSpruce/Can someone teach me how Rathjen’s OCF works?/@comment-35470197-20190723090522/@comment-35470197-20190723220510

> But how do those functions collapse the cardinals?

They collapse cardinals in the same way as other OCFs, i.e. take the least element which does not belong to the set of elements "describable" from below.

> fundamental sequences

When χ appears as an index of ψ, it is regular. If it is singular, then just take the fundamental sequence of the second variable.

Φ admits a similar fundamental sequence as φ, because they are defined in a parallel way.

The fundamental sequence of ψ is very difficult. You need to write down an alogorithm to check cofinality. If you do not know cofinality well, it is good to learn extended Buchholz's OCF.