Question: Say that a variable A in CFL G is usable if it appears in some derivation of some string w G. Given a CFG

Say that a variable A in CFL G is usable if it appears in some derivation of some string w ∈ G. Given a CFG G and a variable A, consider the problem of testing whether A is usable. Formulate this problem as a language and show that it is decidable.

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