1.88 † If An is a countable set for each n E N, show that A = U An, n=1 is again a countable set. (Hint: Explain why each set An can be written in the form An = {ank | k E N} but these sets need not be disjoint from one another. Consider an array similar to that displayed in the proof of Cantor's Theorem in this section, but reason in a manner similar to that in Example 1.14.)

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