 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. HenceThe 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.