User blog comment:P進大好きbot/Proposal to Choose an Official Standard OCF in This Wiki/@comment-34193315-20191220032726/@comment-11227630-20191220042019

The most simple OCF is Taranovsky's Degrees of Recursive Inaccessibility, with an associated ordinal notation.

It has two "simple" points.
 * 1) No "+" is used. Addition is associative and thus multivariable in standard forms, so it makes complication.
 * 2) Term comparisons do not refer Skolem Hull. What refers Skolem Hull is only the determination of standard forms.