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$\{g\in\mathtt{CFG}(\{a,b\})\mid\mathcal{L}(g)=\{a,b\}^*\}\quad\leq\quad\{G\mid\mathcal{L}(G)=\{a,b\}^*-\{a\}\}$
Reduce the universality problem on CFGs over $\{a,b\}$ to the problem of
whether a CFG generates all words over $\{a,b\}$ except the word $a$, in order
to prove that such problem is not semi-decidable (not recursively enumerable).
Authors: Carles Creus, Guillem Godoy
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