= Solution
<List concatenation> exhausts one inner <list> before reaching the next. For example, with rows `[0,2..]` and `[1]`, `concat` outputs the infinite even row and never reaches `1`. <First-occurrence duplicate removal> cannot recover a value that its input stream never supplies. Hence
$$
\boxed{\text{ordinary concatenation does not give a fair enumeration of infinite rows}.}
$$
The obstruction is separate from the undefined final tail in part (c): here a required distinct value is never emitted at all. A <fair enumeration> must give every row and every element within it a finite opportunity to contribute.
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