Example Context free grammar Is there a nice way to give context free grammar for
$$\{a^nb^ma^kb^l:n+m=k+l\}?$$

From PDA point of view it seems we just push + on stack if we see a, push + on stack if we see b, pop + from stack if we see a, pop + from stack if we see b and accept if you have empty stack.
Here is a simple grammar:
$$
\begin{align*}
&amp; S \to aSb|U|V \\
&amp; U \to aUa|T \\
&amp; V \to bVb|T \\
&amp; T \to bTa|\epsilon
\end{align*}
$$