Theorem 6.1.4: the context-free languages are closed under substitution, and hence also under homomorphism.
Theorem 6.1.5: if L ___* is a context-free language and M = (SM, _, _, _M, r, s0), is a DGSM, then M(L) = {r(s0, x)| x_L} ___* is context-free.
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