User blog comment:MilkyWay90/Help with ordinals/@comment-35470197-20180709220316/@comment-31966679-20180717200644

\(\varphi(0_0, 0_1, \ldots, 1_{\alpha + 1}) = \sup_\prec\{\varphi(0_0, 0_1, \ldots, x_\alpha): x \in \text{On}\}\)

What is sup?