Rich Nathan's "Who is My Enemy?" and set theory

Read this section of Rich Nathan's "Who is My Enemy?"  It has to do with set theory, which we have already introduced.  Post a paragraph response in the comments below. (If you don't have one of the accounts listed there, just sign in as "anonymous"..but be sure to include your name in the post so you get credit)  Mention one or two specific things that were interesting to you, or dovetailed with other class learnings,

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