Given the set:
$$\{x | x \in \{a, b\}*~\text{AND}~|x| = 4~\text{AND}~\exists y \in \{a,b\}* : (x = aya)\}$$
Why does the answer look like this: $\{aaaa, aaba, abaa, abba\}$? What I don't understand is why $bbbb$ isn't part of the answer? Does $a$ means $x$ and $y$ means $b$? Could someone just translate the given set in plain English? Thanks in advance!
y
mean? $\endgroup$