\begin{frame}{Top-down Parsing}
  \begin{goal}{}
    \emph{Top-down parsing} tries to derive the input word
    from the starting variable $S$. 
  \end{goal}
  \pause\bigskip

  \begin{block}{}
    \emph{Simple leftmost strategy:}
    \begin{itemize}
      \item 
        Always expand the leftmost variable $A$.\\
        (Replace $A$ by $u$ if there is a rule $A \to u$.) 
      \item
        Backtrack when a mismatch with the input string is found.\\
        (Then try another rule.)
    \end{itemize}
    \pause\smallskip
    \alert{Disadvantage}: backtracking is expensive and difficult.
  \end{block}  
  
\end{frame}