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Say that a variable $A$ in $CFG \:G$ is **necessary** if it appears in every derivation of some string $w \in G$. Let $NECESSARY_{CFG} = \{\langle G, A\rangle \mid \text{A is a necessary variable in G}\}$.

- Show that $NECESSARY_{CFG}$ is Turing-recognizable.
- Show that $NECESSARY_{CFG} $is undecidable.

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