How is it possible to prove whether the language $\{a, b\}^{∗} \setminus \{S → ε, S → aSbS\}$ is context free?
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The first step is finding a simpler description of the language generated by the grammar $S \to aSbS \mid \epsilon$.
The second step is deducing a description of the complement, which is the language you're actually interested in.
Finally, in order to show that the language is context-free, you can consult our reference question on this topic.

Yuval Filmus
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