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\noindent {\it Solu\c c\~ao do Exerc\'{\i}cio 6f:}

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\noindent Gram\'atica que gera $\{wxw^R\,|\, x,w\in\{{\tt a},{\tt b}\}^* \mbox{ e } |w|=|x|\}$.

 \begin{enumerate}
  \item[\ ] $P \,\rightarrow\, {\tt a}XP{\tt a} \,|\, {\tt b}XP{\tt b} \,|\, M$
  \item[\ ] $X{\tt a} \,\rightarrow \, {\tt a}X$
  \item[\ ] $X{\tt b} \,\rightarrow \, {\tt b}X$
  \item[\ ] $X M \,\rightarrow \, M{\tt a}\,|\, M{\tt b}$
  \item[\ ] $M \,\rightarrow \, \lambda$
 \end{enumerate}

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\noindent A vari\'avel $M$ marca o local em que $X$ pode ser substitu\'{\i}da
por ${\tt a}$ ou ${\tt b}$.

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