Let $L1=1^n$;bin$(n),$ where bin$(n)$ is the binary representation of $n.$

Let $L2=$ The language of strings with alphabet $\{a,b,$;$\}$ in the format $x1$;$x2$;dots;$xn$ where each $x$ is a

string of $\{a,b\},$ and each string occurs exactly twice.

Problem $2$

Prove that L$2$ is not context$-$free.