[lang-en]To see the previous post[/lang-en][lang-ko]이전 글 보러가기[/lang-ko] [lang-en]In this post, as commented on the previous post, we shall give a formal definition of ordinal numbers, which are order-types of well-ordered sets. When considering Theorem VI.6, it is natural to think that the order-type of is less than the order-type of if and only if $W_1..