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|     69 \tt Time flies like an arrow. Fruit flies like bananas. |     69 \tt Time flies like an arrow. Fruit flies like bananas. | 
|     70 \end{center}   |     70 \end{center}   | 
|     71  |     71  | 
|     72 \noindent |     72 \noindent | 
|     73 from natural languages were the meaning of \emph{flies} depends on the |     73 from natural languages were the meaning of \emph{flies} depends on the | 
|     74 surrounding \emph{context} are avoided as much as possible. |     74 surrounding \emph{context} are avoided as much as possible. Here is | 
|         |     75 an interesting video about C++ being not a context-free language | 
|         |     76  | 
|         |     77 \begin{center} | 
|         |     78 \url{https://www.youtube.com/watch?v=OzK8pUu4UfM} | 
|         |     79 \end{center}   | 
|     75  |     80  | 
|     76 Context-free languages are usually specified by grammars. For example |     81 Context-free languages are usually specified by grammars. For example | 
|     77 a grammar for well-parenthesised expressions can be given as follows: |     82 a grammar for well-parenthesised expressions can be given as follows: | 
|     78  |     83  | 
|     79 \begin{plstx}[margin=3cm] |     84 \begin{plstx}[margin=3cm] |