I'm currently working through Analytic Combinatorics (free online) and i'm stuck at a very interesting example. After introducing some admissible constructions (combinatorial sum, cartesian product, sequence, powerset, multiset) and talking about how to define structures recursively, there's an example I can't really wrap my head around.
Here's the part i don't understand (page 35 in the pdf):
The book uses $\{\epsilon\}$ to denote the neutral class consisting of a single element of size zero. Could someone explain how this fits into the recursive definition? Without the term $\{\epsilon\}$ it would have made sense to me.
Thanks a lot!