Write a CFG (which will be ambiguous) generating the words of the form $a^{n_0}ba^{n_1}b\ldots a^{n_{m-1}} b a^{n_m}$, with $m\geq 1$, for which $n_0$ is equal to the sum of a selection of naturals from $n_1,n_2,\ldots,n_m$, i.e., $n_0=\sum_{i\in I} n_i$ where $I\subseteq\{1,\ldots,m\}$. Note that, in particular, the selection might be empty, and therefore a word where $n_0$ is $0$ is necessarily correct.